Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

Grace Lu; Amanda J Haes; Tori Z Forbes

doi:10.1016/j.ccr.2018.07.010

. Author manuscript; available in PMC: 2019 Feb 1.

Published in final edited form as: Coord Chem Rev. 2018 Jul 31;374:314–344. doi: 10.1016/j.ccr.2018.07.010

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

Grace Lu ¹, Amanda J Haes ^1,^*, Tori Z Forbes ^1,^*

PMCID: PMC6358285 NIHMSID: NIHMS1001455 PMID: 30713345

Abstract

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Keywords: Uranium, Vibrational spectroscopy, Infrared, Raman

1. Introduction to uranium chemistry

Uranium, element number 92, is one of the most abundant radioactive metals and possesses a troubled history with regard to both its use in the energy and defense sectors and its impact on public health and environmental systems. The most abundant (99.3%) uranium isotope, ²³⁸U, is a primordial radionuclide with a half-life of 4.7 × 10⁹ years and is found in the earth crust at an average concentration of 2.7 ppm [1,2]. Other isotopes, such as ²³³U, ²³⁴U, ²³⁵U, and ²³⁶U, occur at concentrations below 1% of natural abundance although their specific activity is higher than that of ²³⁸U. Due to its fissile nature, enriched uranium (3–5% ²³⁵U) is widely used in nuclear reactors for power generation and higher levels (20–90% ²³⁵U) are used for the development of nuclear weapons [2]. The enrichment process combined with mining and milling of natural uranium ore bodies generates radioactive waste. Additional waste is added after use in a nuclear reactor, where the solid product still contains 95% U that is now mixed with fission products and transuranic materials [3]. Characterizing the chemical nature of waste forms is important for reprocessing and/or long-term storage in a geologic repository so that impacts on the biosphere are minimized [4]. In addition, there are many sites around the world where abandoned uranium mines, tailing piles, or legacy waste are currently in need of remediation to reduce the risk to environmental systems or the public [5].

The complex chemistry of uranium dictates how the element behaves in natural waters, which will in-turn, impact transport processes and bioavailability. While uranium can exist in oxidation states between +3 and +6, the most common states in natural systems are +4 and +6 [6]. U(IV) is the dominate state in slightly oxidizing to anoxic conditions and typically forms insoluble (0.01 μg/L) hydrolysis and microbial mediated oxide products in natural waters [7,8]. These solids contain the U(IV) cation coordinated to 6–10 O atoms and possess interesting catalytic, semiconducting, and magnetic properties [9–11]. U(VI) exists in aerobic conditions and is relatively soluble in natural waters [8]. Bonding is quite unusual in U(VI) as it possesses strong, covalent bonds to two axial O atoms, creating the uranyl cation, [U(VI)O₂]²⁺ [12]. Additional bonding to the uranyl moiety by four, five, or six equatorial ligands forms an overall coordination geometry of square, pentagonal, or hexagonal bipyramidal about the metal center [12]. U(VI) is also considered a hard Lewis acid and prefers to bond to O and N functional groups thus forming a range of soluble complexes and mineral phases in natural waters [8]. Speciation influences the overall solubility of U, ranging from 1 μg U/L for groundwater in equilibrium with vanadates to 120 mg U/L in the presence of carbonates and silicates [13]. Due to the complex chemistry of U(VI), important species for environmental systems can exist as soluble coordination complexes [8,14] and nanoclusters [15,16], colloids [17,18], amorphous precipitates [19,20], or surface adsorbed phases [21]. Pentavalent U(V) is often not mentioned in the discussion of environmental uranium chemistry because it is considered extremely unstable and readily disproportionates to U(IV) and U(VI) [22]; however, it has been observed as a stable phase on mica surfaces [23] and more recently been detected in a catalytic transformations of iron oxyhydroxide mineral phases [24–26].

Given the complexity of uranium chemistry in natural waters, it is important to develop tools to assess speciation, characterize solid phases, understand surface processes, and probe chemical mechanisms. Radiometric techniques, such as alpha spectrometry, quantify U in natural systems down to 0.22 mBq/L and provide isotopic ratios of the materials [27]. These methods, however, do not reveal chemical information on speciation or bonding. Powder and single X-ray diffraction techniques can detect and identify solid-state phases, but require milligram quantities of the material [28,29]. Higher-energy X-ray sources, such as those found at synchrotron facilities, overcome this limitation but require specialized instrumentation and dedicated staff support [30]. Mass spectrometry is more widely used and provides quantitative analysis along with some speciation information [31,32]. Isotopic effects and complex matrices, however, lead to difficulties in data analysis. There are also a wide range of spectroscopic techniques that have been used to characterize uranium compounds and solutions, including fluorescence [33–35], X-ray photoelectron [36], X-ray absorption [18,37,38], and vibrational spectroscopy [39]. Each of these techniques provides different chemical information regarding the complexation, coordination environment, bonding, and photoelectric properties although some are limited to either solution or solid phase (Table 1).

Table 1.

Instrumentation to assess the chemistry of uranium in solution and solid-state samples.

	Phase	Chemical Speciation/Coordination	Isotopic Ratios	Detection Limit/Dynamic Range	Complex Matrix Compatibility
Alpha Spectrometry	Solution	Poor	Excellent	0.05 pCi/-	Poor
Liquid Scintillation Counter	Solution	Poor	Poor/Good	50 pCi/-	Poor
X-ray Diffraction	Solid	Excellent	Poor	1%/-	Poor
X-ray Photoelectron Spectroscopy	Solid	Good	Poor	0.1–1%/-	Poor
X-ray Absorption Spectroscopy	Solution/solid	Excellent	Poor	1 × 10⁻⁶M/-	Good
X-ray Scattering	Solution	Excellent	Poor	1 × 10⁻³ M/-	Fair
Mass Spectrometry	Solution	Good	Excellent	1 × 10⁻¹¹ M/-	Poor
Fluorescence	Solution/solid	Excellent	Poor	5 × 10⁻⁶ M/5+	Good
Time-Resolved Fluorescence Spectroscopy	Solution	Excellent	Poor	1 × 10⁻⁷ M/5+	Good
UV-Vis Absorption	Solution/solid	Poor	Poor	1 × 10⁻⁶M/3+	Good
Infrared Absorption	Solution/solid	Good	Poor	1 × 10⁻⁷ M/3+	Good
Raman Scattering	Solution/solid	Excellent	Poor	1 × 10⁻⁵ M/2+	Excellent

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One of the most versatile and wide-spread spectroscopic methods is based on measuring vibrational energies and encompasses both infrared (IR) absorption and Raman scattering. These complementary spectroscopic methods reveal information regarding bonding and local environment for compounds and species, including those for uranyl. While both methods rely on excitation of quantized vibrational-rotational energy levels in these samples, observed spectral features depend on unique selection rules. For infrared spectroscopy, a change in dipole moment must arise upon vibrational motion for the mode to be IR-active while a change in polariz-ability (i.e., distortion of the electron cloud) must occur similarly for a mode to be Raman-active. The asymmetric (ν₃) and symmetric (ν₁) uranyl stretches are the most probable and intense modes in IR and Raman spectra, respectively, and both are sensitive to coordinating ligands, local environment, and molecular geometries. These modes occur at different frequencies, however, because they require different amounts of energy to excite vibrational motion. Because water vibrational modes are silent or weak in Raman spectra, Raman spectroscopy is widely utilized for characterizing environmental and aqueous-phase uranyl samples, but the signals for uranyl are weak because of the probability of exciting the Raman-active modes is low. In comparison, water exhibits strong IR bands that interfere with uranyl detection from these samples unless attenuated total reflectance (ATR) cells are used. This approach works best for solid samples that can adhere to the ATR crystal so that the spectroscopic signal from water can be minimized. The signals are more intense in comparison to Raman spectral features but often exhibit a large background and/or features that overlap with other chemical species in the samples. Advanced vibrational spectroscopic methods have been developed for solution phases, solid materials, and surfaces [40,41]. Traditionally, one of the limitations of these methods was poor detection limits; but in more recent years, new technologies have been employed to enhance detection and provide additional chemical information.

The purpose of this review is to provide an overview of how uranium can be characterized using vibrational spectroscopy, including IR absorption and Raman scattering. Previous literature examinations focused on the detection and identification of uranium speciation using X-ray spectroscopy [42,43], time-resolved laser-induced fluorescence spectroscopy [44–46], and computational modeling [47]. An excellent review of IR vibrational frequencies of uranyl (VI) mineral compounds was also provided by Cejka in 1999, but the reported uses of vibrational spectroscopy for uranium chemistry has grown in number and significant advances in sample preparation and data analysis have increased in both breadth and depth; therefore, there is a critical need to provide an update on this important chemical technique [39]. We have also published a study investigating the best practices for collecting and analyzing Raman spectra in aqueous solutions, but determined that additional information was lacking in regards to the analysis of solid state samples and surfaces [48]. Building on the previous work by Cejka [39], the current review focuses on how both IR and Raman spectroscopies are used to characterize solid state uranium compounds; identify U adsorbed on surfaces; and provide speciation information within aqueous solutions, organic solvents, and ionic liquids. In addition, we explore the latest advances in uranium detection and provide an analysis of future needs and unanswered questions.

2. Detection and identification of uranium using vibrational spectroscopy

Vibrational spectroscopy is used to elucidate uranium speciation because each species possesses a unique vibrational frequency, which is influenced by the valence number, bond length, coordination ligands, structure, and local environment. Correct analysis of these complex data is of utmost importance. Careful analysis requires an understanding of (1) appropriate spectral windows relevant for uranyl speciation determination, (2) implications of possible vibrational band overlap arising from multiple uranyl species, and (3) impacts of coordinating ligands and phase on vibrational band widths. In the following section, we provide an overview of the overall spectral characteristics of uranium in different valence states and summarize additional information on proper data analysis.

2.1. Vibrational spectroscopy of U(IV)

U(IV) solids have variable coordination numbers, and UAO bonds contain substantial ionic character, leading to smaller vibrational mode cross sections and less intense vibrational bands. UO₂ solid crystallizes in the fluorite Fm3m space group, and group theory predicts one IR and one Raman active mode [49]. With hyper-stoichiometric U(V) oxides, the fluorite lattice often distorts, leading to relatively lower symmetry structures and activation of more vibrational modes [49,50]. For instance, β-U₄O₉ forms in the I-43d space group with 7 crystallographically unique U and 14 O atoms [50]. Modeling suggests that modes consistent with UO₂ and phonon lattice vibrations could arise in Raman analysis thus increasing the number of possible vibrational modes observed in this structure type by a factor of four [49]. Vibrational band broadening may also occur with increasing disorder within the crystalline lattice [50]. Based upon solubility limitations and the weak vibrational bands, the use of vibrational spectra in U(IV) solutions is rather limited. Often the use of absorption spectroscopy is more conducive for confirming the presence of U(IV) whereas vibrational spectroscopy is appropriate for identifying other inorganic or organic anions with active bands [51].

2.2. Vibrational spectroscopy of U(VI)

In contrast to the tetravalent state, U(VI) forms the uranyl cation, $[U (VI) O_{2}^{2 +}]$ and displays strong active vibrational bands associated with the covalent axial bonds. There are three fundamental modes of vibration for the uranyl moiety (D_∞h): the sym metric stretch (ν₁), bend (ν₂), and antisymmetric stretch (ν₃) [39] (Fig. 1). The bending mode occurs in two mutually perpendicular planes making it degenerate. When the D_∞h symmetry is maintained, the ν₁ is Raman active, and ν₂ and ν₃ are IR active [39]. Symmetry lowering (D_∞h → C_∞v, C_∞v, C_s) occurs when the two axial bonds within the uranyl moiety are not equal in length or through significant bending (>5°) of the linear dioxo bond [52,53]. Decreasing the symmetry of the uranyl moiety should result in the activation of all three fundamental modes in IR and Raman spectra.

Fig. 1. — Vibrational modes for the [UO₂]²⁺ cation and frequencies for the [UO₂(H₂O)₅]²⁺ species. The uranyl pentaqua species in the D_∞h point group typically serves as a benchmark for determining a red of blue shift of these uranyl vibrational modes.

The symmetric and asymmetric stretching modes are both informative and sensitive to coordinating ligands and structure (e.g., mono-, bi-, or tridentate chelation). This occurs because the uranyl bond is weakened by additional coordination in the equatorial plane [54]. Perturbation of the uranyl bond can be explained by either the presence of strong electron donating ligands occupying the equatorial coordination sites or destabilization from purely electrostatic interactions [55]. Weakening of the uranyl bond results in bond elongation, causing the uranyl vibrational frequency to red-shift from the widely-used comparative standard, the pentahydrate species $({UO}_{2} {(H_{2} O)}_{5}^{2 +})$ . The symmetric and asymmetric stretching bands for ${UO}_{2} {(H_{2} O)}_{5}^{2 +}$ is commonly observed at 870 and 962 cm⁻¹, respectively [48]. This leads to a typical spectral window for U(VI) compounds ranging from 900 to 750 cm⁻¹ for Raman and 980–830 cm⁻¹ for IR spectroscopy.

The Raman-active bands can be fit to a Gaussian function, and the full width at half-maximum (Γ) of these bands is typically between 13 and 15 cm⁻¹ [48]. Vibrational band frequencies are observed for monomeric, dimeric, and trimeric hydrolysis products. For instance, a ~20–30 cm⁻¹ shift in vibrational frequency of the symmetric stretch has been reported as oligomeric species are generated by hydrolysis [56]. This shift is also accompanied by slight broadening of the bands (ΔΓ = 5 cm⁻¹) due to the formation of these larger soluble clusters [48].

An example of Raman data analysis for speciation determination is demonstrated in Fig. 2. Here, Raman spectra of uranyl nitrate crystals (used as received from International Bio-Analytical Industries, Inc. Lot #55971) were collected using a 785 nm laser to reduce the effects of elastic scattering and fluorescence interference on the background in the uranyl window (950–700 cm⁻¹). Raw spectral data are shown in panel B. Next, inverse second derivative spectra are generated and plotted so that approximate vibrational band centers can be noted. Bands were considered significant if band widths were greater than 15 cm⁻¹ and larger than 5% of the noise. As shown, four significant features were noted and were centered at 874, 865, 754, and 715 cm⁻¹. These vibrational band centers with ±3 cm⁻¹ windows and 15–45 cm⁻¹ widths were used for subsequent fitting using Lorentzian functions. In so doing, four bands were identified at 875 (v₁ U=O; uranyl nitrate dihydrate) [57], 868 (ν₁ U=O; uranyl nitrate hexahydrate) [58], 754 (ν₃ (A₁), NO), and 714 (ν₅ (B₁), NO) cm⁻¹ [59]. Note that the m₁ band for the dehydrate material is slightly blue-shifted compared to the original pentaqua uranyl species whereas the hexahydrate compound exhibits a 2 cm⁻¹ red-shift compared to the same benchmark complex. The splitting of the band associated with the uranyl symmetric stretch, also allowed us to determine that the compound was composed of a biphasic mixture with differences in hydration state.

2.3. Vibrational spectroscopy of U(V)

While U(V) complexes are quite unstable, the presence of this moiety in solution or solid state can be detected using vibrational spectroscopy. Pentavalent uranium also forms the uranyl cation, but the charge is reduced to [U(V)O₂]⁺. Additional coordination about the equatorial plane leads to a similar coordination environment as with U(VI) species. The major difference with the two valence states is the elongation of the U(V)=O bond to 1.808–1.916 Å and a red shift in the vibrational modes [60]. The oxo groups of the $U (V) O_{2}^{+}$ species are more reactive than the hexavalent moiety and interact with other cations or actinyl groups [61]. This interaction should promote elongation of one of the uranyl bonds, resulting in lower molecular symmetry. The consequence of asymmetric uranyl bonds within the vibrational spectra is similar to what is reported for U(VI) species, where all three fundamental modes can be activated in both the IR and Raman spectra [39].

3. Chemical and structural elucidation of uranium solid-state compounds

Vibrational spectroscopy has been used for over 80 years to identify and understand the structural features of simple inorganic U salts and mineral phases [62]. This is particularly true for mineral phases, and we would be remiss if we did not acknowledge the amazing body of literature provided by Ray L. Frost, Jiri Cejka, and their research groups/colleagues. In the last 10 years, spectroscopic analysis of uranyl hybrid materials significantly expanded due to an interest in understanding the uranyl moiety bond strength and providing additional characterization to enhance the structural description of novel compounds. In the next section, we analyze the spectral signals for uranium compounds with extended topologies and mineral phases before exploring uranium coordination compounds and hybrid materials. In addition, we use a subset of well-characterized compounds to provide updated metrics for the analysis of bond lengths and predicted spectroscopic signals.

3.1. Mineral phases and inorganic uranium compounds with extended topologies

Some natural mineral specimens contain chemical and structural variations based upon the specific geologic conditions that influence vibrational band intensities and frequencies. To simplify the discussion of these compounds, we focus on the major bands associated with the uranyl cation, and the reader should refer to the primary literature to evaluate subtle spectral differences in these data. Relationships between vibrational band frequencies and bond lengths, which are based upon the empirical formula derived from Bartlett and Cooney [63] are discussed herein. We summarize the major vibrational modes within the text and provide detailed in Table 2. We would like to note that the values included in the tables are those directly reported in the literature and not our own interpretation. In addition, the Raman bands are emphasized in this section because of difficulties in identifying the ν₃ mode in IR spectra due to significant band overlap from interfering species. When possible, we obtained information on the asymmetric stretch and included those values in the tables.

Table 2.

Vibrational modes for solid state inorganic compounds and mineral phases for hexavalent uranium compounds.

Compound (Mineral Name)	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	Refs.
Oxides, Oxyhydroxides, Peroxides, Halides
(Ca,U(VI))O₄ (Vorlanite)	683		[84]
[(UO₂)₈O₂(OH)₁₂]·12H₂O (schoepite)	838		[86]
K₂[(UO₂)₃O₂(OH)₃]₂·7H₂O (compreignacite)	848, 824		[273]
K₂[(UO₂)₃O₂(OH)₃]₂·7H₂O (compreignacite)	831		[87]
Ca[(UO₂)₆O₄(OH)₆]·8H₂O (becquerelite)	838, 813		[86]
Ca[(UO₂)₆O₄(OH)₆]·8H₂O (becquerelite)	829, 796	948, 872	[87]
Ba[(UO₂)₆O₄(OH)₆](billietite)	830		[86]
Pb[(UO₂)₁₀O₆(OH)₁₁](vandendriesscheite))	852, 840		[86]
Pb_3+x(H₂O)[(UO₂)₄O_4+x(OH)_3x]₂ (curite)	803, 791	912, 876	[86,88]
Cu[UO₂(OH)₄] (vandenbrandeite)	805	897	[89]
K₂Ca[(UO₂)₆O₆(OH)₄]·6H₂O (rameauite)	813		[274]
KPb[(UO₂)₇O₅(OH)₇]·8H2O (gauthierite)	833	915	[275]
[N(C₂H₅)₄]₂[(UO₂)₄(OH₂)₃F₁₀]	845	927	[276]
[(UO₂)(O2)(H₂O)₂]·2H₂O (studtite)	831		[95]
UO₂F₂	867	915	[100]
UO₂Cl₂	871	960	[101]
Carbonates
Ca₂[UO₂(CO₃)₃]·11H₂O (liebigite)	822		[277]
Ca₂[UO₂(CO₃)₃]·11H₂O (liebigite)	829	906	[87]
Ca₂ [UO₂(CO₃)₃]·10H₂O (synthetic)	826		[278]
Mg₂ [UO₂(CO₃)₃]·18H₂O (bayleyite)	822		[278]
Na₄[(UO₂(CO₃)₃] (cejkaite)	805		[279]
Na₄[(UO₂(CO₃)₃] (synthetic)	810, 816		[279]
Na₂Ca(UO₂)(CO₃)₃·6H₂O (andersonite)	806, 832	899, 913	[87]
Na₂Ca(UO₂)(CO₃)₃·6H₂O (andersonite)	832	898	[253]
K3Na(UO₂)(CO₃)₃·H₂O (grimselite)	815	876	[87]
Sr₂[UO₂(CO₃)₃]·8H₂O	812		[278]
Ba₂[UO₂(CO₃)₃]·6H₂O	818		[278]
K₄UO₂(CO₃)₃	806		[280]
Pb₂(UO₂)(CO₃)₃ (Widenmannite)	849	926	[281]
(UO₂CO₃) (Rutherfordine)	886	956	[282,283]
(UO₂CO₃) (Rutherfordine)	886	932	[284]
NaCa₃(UO₂)(CO₃)₃(SO₄)F·10H₂O (schrockingerite)	817	898	[285]
Ca[(UO₂)(CO₃)₂(H₂O)₂]·3H₂O (zellerite)	834	899	[286]
Ca₂Cu[(UO₂)(CO₃)₃](CO₃)·6H₂O (voglite)	836	898	[287]
Y₂(UO₂)₄(CO₃)₃(OH)₈·10–11H₂O (kamotoite-Y)	814.7	911	[288]
CaU⁵⁺(UO₂)₂(CO₃)O₄(OH)(H₂O)₇ (Wyarite)	853, 837		[124]
Sulfate and Selenate
UO₂SO₄ 2.5H₂O	863,853	952, 938	[289]
UO₂SO₄ 3.5H₂O	865		[290]
(H₃O)₂[(UO₂)₂(SO₄)₃(H₂O)]·7H₂O		930	[291]
(H₃O)₂[(UO₂)₂(SO₄)₃(H₂O)]·4H₂O		918, 928, 935	[291]
[(UO₂)₆(SO₄)O₂(OH)₆(H₂O)₆](H₂O)₈ (uranopilite)	843, 835, 819	941, 929, 910	[292,293]
(UO₂)₈(SO₄)(OH)₁₄·13H₂O (jachymovite)	839, 828, 807, 800	902, 923	[294]
K_0.6(H₃O)_0.4[(UO₂)₆(SO₄)₃(OH)₇]·8H₂O (zippeite)	849, 838, 826, 814	911, 873	[295]
Na₄[(UO₂)₆(SO₄)₃(OH)₁₀]·4H₂O (Na zippeite)	840, 841, 833, 823	880, 912	[296]
Na₅(UO₂)₈(SO₄)₄O₅(OH)₃·8H₂O (natrozippeite)	840		[109]
Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O(johannite)	811		[297]
Cu(UO₂)₂(OH)₂(SO₄)₂·8H₂O(johannite)	836		[109]
Cu_6.5[(UO₂)₄O₄(SO₄)₂]₂(OH)₅·25H₂O (pseudojohannite)	810,805		[298]
Fe(UO₂)(SO₄)₂(H₂O)₁₁ (leydetite)	858, 851, 846, 843, 836, 828	937, 930	[299]
Fe[(UO₂)₂(SO₄)₂(OH)₂](H₂O)₇ (deliensite)	838	933	[300]
(UO₂)₈(SO₄)(OH)₁₄·13H₂O (jachymovite)	839, 828, 807, 800	902, 923	[294]
Na₅(UO₂)(SO₄)₃(SO₃OH)(H₂O)(meisserite)	847		[301]
Na₇(UO₂)(SO₄)₄Cl(H₂O)₂ (bluelizardite)	854,848		[302]
K(UO₂)(SO₄)(OH)(H₂O) (adolfpateraite)	843	890	[303]
Na₆[(UO₂)(SO₄)₄](H₂O)₄	860, 845, 830, 805		[304]
Fe(UO₂)(SO₄)₂·5H₂O (rietveldite)	862	924	[305]
[(UO₂)(SO₄)(H₂O)₂]₂·H₂O (shumwayite)	865,850	951, 927	[306]
(UO₂)₃(SeO₃)₂(OH)₂·5H₂O(haynesite)	811, 800	905, 862	[307,308]
Cu[(UO₂)₃(SeO₃)₂O₂]·8H₂O (marthozite)	812	863	[309]
Cu₄UO₂(SeO₃)₂(OH)₆·H₂O(derriksite)	788	859	[310]
Pb₂Cu₅(UO₂)₂(SeO₃)₆(OH)₆·2H₂O (demesmaekerite)	822	878	[311]
Ba[(UO₂)₃O₂(SeO₃)₂](H₂O)₃ (guilleminite)	831	912	[312]
(Y_1.98Dy_0.24)_2.22H_0.34[(UO₂)₈O₈₈O₇OH(SO₄)₄](OH)(H₂O)₂₆ (sejkoraite-Y)	829	911	[313]
Phosphate and Arsenate
UO₂(H₂PO₂)₂·H₂O	830	909	[314]
UO₂(H₂PO₂)₂·H₃PO₂	830	912	[314]
(H₃O)(UO₂)PO₄·4.5H₂O	842		[315]
K_0.9(H₃O)_0.1(UO₂)PO₄·2.5H₂O	826		[315]
K_0.4(H₃O)_0.6(UO₂)PO₄·3H₂O	840		[315]
Cs₃(UO₂)₂(PO₄)O₂		885, 855, 838, 805	[316]
Ca(UO₂)₂(PO₄)₂·11H₂O (autinite)	830		[108]
KCa(H₃O)₃(UO₂)₇(PO₄)₄O₄·8H₂O] (phosphuranylite)	827		[108]
KCa(H₃O)₃(UO₂)₇(PO₄)₄O₄·8H₂O] (phosphuranylite)	832	894	[115]
Ca(UO₂)₃(PO₄)₂(OH)₂·6H₂O (phosphuranylite)	801		[109]
Ca₂(UO₂)₃O₂(PO₄)₂·7H₂O (phurcalite)	810	874	[317]
[Mg(UO₂PO₄)₂·10H₂O] (saleeite)	837	901	[108]
[CaCu(UO₂)(PO₄)₂·4H₂O] (ulrichite)	812	880	[108]
[Cu(UO₂PO₄)₂·8H₂O] (metatorbernite)	826	906	[108]
	813	876–902
	840
(H₃O)_0.4Cu_0.8(UO₂)₂(PO₄)₂·7.6H₂O (metatorbernite)	822	895	[318]
Cu(UO₂)₂(PO₄)₂·12H₂O (torbernite)	825		[109]
[(Ba)₂(UO₂)₃(PO₄)₂(OH)₄·8H₂O] (bergenite)	810,798		[319]
(H₃O)Al(UO₂)₄(PO₄)₄·15H₂O (sabugalite)	850, 836, 826, 810		[110]
Al[(UO₂)₂(PO₄)₂](OH)·8H₂O (threadgoldite)	827		[320]
Pb₂(UO₂)(PO₄)₂·n H₂O (parsonsite)	807, 796		[321]
Pb₃[H(UO₂)₃O₂(PO₄)₂]₂·12H₂O(dewindtite)	831, 823, 818, 808, 795		[322]
Pb₂[(UO₂)₃O₂(PO₄)₂]·5H₂O(dumontite)	815,800, 780		[323]
(UO₂)Bi₄O₄(PO₄)₂·2H₂O (phosphowalpurgite)		885	[324]
Nd[(UO₂)₃O(OH)(PO₄)₂]·6H₂O (francoisite-Nd)	830	934	[325]
U(OH)₄[(UO₂)₃(PO₄)₂(OH)₂]·4H₂O (vanmeersscheite)	860	884	[326]
(Na₂,Ca)[(UO₂)(AsO₄)]₂·5H₂O (natrouranospinite)	816, 810	917	[327]
Ca[(UO₂)(AsO₄)]₂·8H₂O (metauranospinite)	815, 806	904, 893	[328]
Ca(UO₂)[(UO₂)₃(AsO₄)₂(OH)₂]·(OH)₂·6H₂O (arsenouranylite)	795, 787		[326]
Mg(UO₂)₂(AsO₄)₂·10H₂O (novacekite)	817		[109]
Cu(UO₂)₂(AsO₄)₂·12H₂O (zeunerite)	821		[109]
Cs₂[(UO₂)(As₂O₇)]	832		[329]
α-Cs[(UO₂)(HAs₂O₇)]	834		[329]
β-Cs[(UO₂)(HAs₂O₇)]	820		[329]
Cs[(UO₂)(HAs₂O₇)]·0.17H₂O	809	896	[329]
Ni(UO₂)₂(AsO₄)₂·8H₂O (metarauchite)	817	947	[330]
(UO₂)Bi₄O₄(AsO₄)₂·2H₂O (walpurgite)	790, 770	888	[331]
(zn_0.72Fe_0.10Mg_0.06Al_0.05)(UO₂)₂[(AsO₄)_1.5(PO₄)_0.5]₂·8.43H₂O (metalodevite)	819	891	[331]
Silicates
(UO₂)₂SiO₄·2H₂O (soddyite)	832, 828		[233]
	830	897	[332]
	828, 809, 802	904, 910, 900	[333,334]
Na[UO₂SiO₃(OH)]·0.5H₂O (sodium boltwoodite)		848	[335]
K[UO₂SiO₃(OH)]·H₂O (boltwoodite)		841	[335]
(K,Na)[(UO₂)(SiO₃OH)]·1.5H₂O (boltwoodite)	799, 796, 786	853	[336]
K₂[(UO₂)₂(Si₅O₁₃)]·H₂O (weeksite)	814, 810, 800	916, 903	[333,334]
Ca(UO₂)₂(SiO₃OH)₂·5H₂O (uranophane)	799, 796, 799	856, 878, 852	[337]
		855	[335]
	800		[338]
	800		[109]
	798		[233]
Mg(UO₂)₂(SiO₃OH)₂·5H₂O (sklodowskite)	827, 801, 777	854, 821	[336]
Cu(UO₂)₂(SiO₃OH)₂·6H₂O (cuprosklodowskite)	787	871	[336]
Cu(UO₂)₂(SiO₃OH)₂·6H₂O (cuprosklodowskite)	792		[109]
PbUO₂SiO₄·H₂O (kasolite)	759	904	[336]
	768		[109]
Ca[(UO₂)₂(Si₅O₁₂(OH)₂](H₂O)₃ (haiweeite)	808, 800	909, 877	[333,334]
Iodates and Tellurates
UO₂(IO₃)₂(H₂O)·2HIO₃		902	[339]
Na₂[UO₂(IO₃)₄(H₂O)]		903	[340]
K₂[(UO₂)₂(VO)₂(IO₆)₂O]·H₂O	862	982	[341]
K₂[UO₂(MoO₄)(IO₃)₂]		909	[342]
K[UO₂Te₂O₅(OH)]		860	[343]
Tl₃(UO₂)₂[Te₂O₅(OH)](Ie₂O₆)·2H₂O		863	[343]
α-Tl₂[UO₂(TeO₃)₂]		843	[343]
Sr₃[UO₂(TeO₃)₂](TeO₃)₂		847	[343]
[UO₂TeO₃] (schmitterite)	823	937, 882	[344]
PbUO₂(TeO₃)₂ (moctezumite)	826	837	[345]
Tungstates, Vanadates, and Chromates
Cs₄[(UO₂)₄(WO₅)(W₂O₈)O₂]	765, 782		[118]
Cs₄[(UO₂)₇(WO₅)₃O₃]	871, 828, 786, 766		[118]
Na₈[TeW₉O₃₃]·19·.5H₂O	796		[346]
(NH₄)₁₄[(UO₂)₂(H₂O)₂(SbW₉O₃₃)₂]·24H₂O	804		[346]
Pb₂(UO₂)(V₂O₈)·5H₂O (carnotite)	837, 826	900, 860	[347]
K₂(UO₂)(V₂O₈)·3H₂O (curienite)	825		[347]
Ba,Pb(UO₂)(V₂O₈)·5H₂O (francevillite)	828		[347]
Ca(UO₂)(V₂O_s)·9H₂O (Tyuyamunite)	829		[347]
Ca(UO₂)(V₂O₈)·3H₂O (metatyuyamunite)	824, 793		[347]
K₅(UO₂)₄(SO₄)₄(VO₅)·4H₂O (mathesiusite)	830	888, 896	[348]
Rb[UO₂(CrO₄)(IO₃)(H₂O)]		914	[342]
K₂[UO₂(CrO₄)(IO₃)₂]		902	[342]
Rb₂[UO₂(CrO₄)(IO₃)₂]		903	[342]
Cs₂[UO₂(CrO₄)(IO₃)₂]		880	[342]

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3.1.1. Uranium oxide, peroxides, hydroxides, and halides

The most abundant and economically important uranium mineral is uraninite (UO₂). This phase is always partially oxidized in environmental systems, which leads to the formation of varying stoichiometries (UO_2+x, U₄O₉, U₃O₇, and U₃O₈) that occur independently or as a corrosion rind on solid materials [64]. For stoichiometric UO₂ with the defect-free fluorite structure, group theory predicts one strong Raman (T_2g) and one weak IR active band (340 cm⁻¹, T_1u). A narrow Raman band is located at 445 cm⁻¹, whereas a broad IR mode is centered at ~340 cm⁻¹ and overlaps with another band at ~470 cm⁻¹ [65–68]. A second weak band at 1150 cm⁻¹ also appears in Raman spectra and was previously assigned by Livneh and Sterer as a second-order longitudinal optical (LO) phonon mode [69]. Hyperstoichiometric amounts of oxygen in uranium dioxide (UO_2.03) leads to the appearance of a broad asymmetric feature with a maximum centered at 560 cm⁻¹ and shoulder at 630 cm⁻¹, corresponding to defect induced degenerate LO modes and anion sublattice distortions, respectively [49,70]. Increasing the O content leads to further broadening and a blue-shift of the T_2g band, a decrease in the LO band intensity, an increased in the intensity of the peak at 630 cm⁻¹, and changes in the relative intensity at 560 cm⁻¹ [49].

Elorrieta et al. [49] utilized Raman spectroscopy to quantitatively characterize the hyperstochiometric material by closely investigating the anion sublattice distortion at 630 cm⁻¹. The value for x within UO_2+x can be quantified using one of the following two equations using the intensities of the modes at 630 and 445 cm⁻¹ (I₆₃₀ and I₄₄₅, respectively):

v_{630} = 654 \pm 4 - (610 \pm 60) x, w h e n \frac{I_{630}}{I_{445}} < 0.24

v_{630} = 647 \pm 4 - (90 \pm 30) x, w h e n 0.27 < \frac{I_{630}}{I_{445}} < 0.24

It is important to note that these equations are valid for the 632.8 nm excitation wavelength because other excitation wavelengths may lead to selective resonant enhancements or high fluorescent backgrounds both of which prohibit quantitative analysis [49].

Partial oxidation to U(IV,V)₃O₇ (also reported as UO_2.3 or tetrag onal UO_2+x) and U(V,VI)₃O₈ results in the appearance of new Raman bands that have been previously used to characterize corrosion of UO₂ fuel pellets [71]. Group theory for D_4h point symmetry predicts 18 phonon branches for the tetragonal U₃O₇ phase, with nine Raman active $(A_{1}^{+} + 2 e_{1}^{+} + 3 Γ^{+})$ and five infrared active $(A_{2}^{-} and 2 Γ^{-})$ modes.[72] Only six of those modes (A_1g + 2 B_1g + 3 E_g) were observed experimentally for U₃O₇ and centered at 630, 155, and ~470 cm⁻¹, respectively [72]. The ingrowth of the B_1g and E_g bands, therefore, are used to determine structural transformations from the cubic to tetragonal phase of UO_2+x. Formation of orthorhombic U₃O₈ results in the characteristic A_2u combination band at 750 cm⁻¹ [73,74]. Other bands associated with U₃O₈ include the A_1g (335 cm⁻¹), A_1g (410 cm⁻¹), and the A_1g (475 cm⁻¹) stretching bands. Precise analysis of these bands during aging of fuel pellets provided growth rates of secondary alteration phases, formation of lattice defects, and inhomogeneities to due irradiation of the material [67,68,72,75–77].

Full oxidation to U(VI) leads to the formation of the uranyl moiety and related spectroscopic signals for the oxide phases. The simplest U(VI) oxide is UO₃, which can form at least seven different crystalline forms depending on the identity of the starting material [78,79]. Of the seven polymorphs, γ-UO₃ is the most studied, with major Raman modes located at 767, 484, and 399 cm⁻¹ [79]. DFT calculations combined with experimental results provide structural details of the gamma phase and indicate that there are two crystallographically unique U atoms [78,80]. U1 is observed in a square bipyramidal coordination geometry with a uranyl bond length of 1.87 Å. The coordination geometry of the U2 site is an unusual dodecahedron with axial bond lengths of 1.78 Å and an O=U=O bond angle of 174.6°. Using the relationship established by Barlett and Cooney [63], the calculated bond length associated with the Raman mode at 767 cm⁻¹ is 1.85 Å, which agrees well with the reported values [80]. Other vibrational modes have not been specifically assigned, but likely arise from uranyl stretching modes. Diuranate compounds (X₂U₂O₇), particularly sodium and ammonium forms, are also important U(VI) oxides because they are precipitated as an intermediate product during the production of yellowcake and nuclear fuel pellets [81]. The main spectroscopic band associated with uranyl in Na₂U₂O₇ is located between 789 and 778 cm⁻¹, whereas the NH₄ form ((UO₂(OH)_2−x(O)(NH₄)_x)·yH₂O) exhibits bands within a larger spectral window (841–804 cm⁻¹) due to hydrolysis and variability in hydration that occurs between synthetic batches [82,83]. Only one other U(VI) oxide phase, the rare mineral Vorlanite (Ca,U(VI)O₄), has been reported in the literature. The major band associated with the uranium cation is observed at 683 cm⁻¹, which corresponds to octahedrally coordinated U(VI) with a bond length of 2.33 Å [84].

Hydrolysis of uranium oxide materials occurs in the presence of water and leads to a range of uranyl oxyhydroxide phases in a complex matrix such as groundwater. Schoepite [(UO₂)₈O₂(OH)₁₂]-·12 H₂O occurs in solutions at near neutral pH and low ionic strength values [85]. The ν₁ mode for this phase occurs at 839 cm⁻¹ with a distinct shoulder at 855 cm⁻¹ and a weak band at 802 cm⁻¹ [86]. With increasing ionic strength, other common uranyl oxyhydroxide minerals form, with the overall general formula of M_n[(UO₂)_xO_y(OH)_z](H₂O)_m, where M = K⁺, Ca²⁺, Pb²⁺, Ba²⁺, and Sr²⁺ [85]. The spectroscopic envelope for most of the oxyhydroxide minerals are observed from 855 to 830 cm⁻¹, with a handful of weak modes appearing at lower wavenumbers [86,87]. Curite (Pb_3+x(H₂O)[(UO₂)₄O_4+x(OH)_3x]₂) and vandenbrandeite (Cu[UO₂ (OH)₄]) are notable exceptions because the uranyl bands are significantly red shifted to ~800–770 cm⁻¹ [86,88,89]. This difference in vibrational frequency arises from structural variations within the mineral sheet topologies. The curite sheet topology is quite different than that of the other uranyl oxyhydroxides, such that it contains U1 in a distorted square bipyramid [90]. Vandenbrandeite is also unique as it contains Cu²⁺ cations bonded to oxo atoms of the neighboring uranyl pentagonal bipyramids [90,91]. In the case of curite, this leads to an elongation of the uranyl bonds (1.79(2)–1.89(2) Å), which induces a significant red-shift in the vibrational band compared to the other oxyhydroxide phase [86,90]. For vandenbrandeite, the uranyl bond distances are closer to the average value (1.77 Å), thus additional investigations are needed to clarify the perturbation of the uranyl bond [91].

The uranyl mineral, studtite, is the only naturally occurring peroxide-bearing phase and widely occurs as a secondary corrosion product of urananite ores and nuclear fuel rods [92,93]. The material contains a chain topology that propagates through edge sharing peroxide bridges [94]. The Raman spectra contains a v₁ uranyl band located at 831 cm⁻¹ and a secondary v₁ (O=O) stretching mode at 870 cm⁻¹ [95]. Both studtite and the dehydrated form (metastudtite [UO₂(O₂)(H₂O)₂]) have been identified as a secondary alteration phase formed due to the alpha radiolysis of water using Raman spectroscopy as the main surface characterization technique [76,77,96,97].

While there are several uranyl halide phases reported in the literature, significant spectral data are only available for UO₂F₂ and UO₂Cl₂. Structural features of the fluoride compound was first established by Zachariasen [98] and then refined with neutron diffraction by Atoji and McDermott [99]. The structure contains sheets of uranyl hexagonal bipyramids, with six edge sharing F atoms located in the equatorial plane and an experimental UAO bond length of 1.74(2) Å. Theoretical studies predict a Raman band at 915 cm⁻¹ and a calculated bond length of 1.71 Å. Hydration of the uranyl fluoride compound induces a red shift in the m₁ mode to 867 cm⁻¹, which agrees well with the calculated bond length of 1.75 Å [100]. Bullock spectroscopically characterized UO₂Cl₂, denoting the ν₁ at 871 cm⁻¹ and ν₃ at 960 cm⁻¹ [101]. The uranyl bond distance reported for the chloride compound is 1.70 Å, which is lower than expected based upon the spectroscopic signal (1.74 Å) [102].

3.1.2. Uranium compounds and minerals containing oxyanions

The chemical complexity of oxyanion-containing U(VI) compounds and mineral phases leads to a wide spectral envelope for the vibrational bands associated with the uranyl moiety. Fig. 3 provides the averaged ν₁ vibrational band frequencies (stars) as well as the range of values for the various oxyanion species. We have chosen to present these published results as box and whisker plots to emphasize the variation in values reported. In particular, these plots visually depict the distribution of results including the mean (x), median (middle vertical line), 25th and 75th quartiles (3rd and 1st vertical lines, respectively), as well as the minimum and maximum values (ends of error bars). Average values are observed between 840 and 800 cm⁻¹ with ranges extending from 870 to 760 cm⁻¹. This large range likely arises because of the complex nature of the extended structural topologies and subtle variations in the coordination environment about the uranyl cation within different compounds and mineral phases. Complexity is enhanced from symmetry lowering within the solid state compounds upon activating the vibrational and splitting degenerate modes. Despite this, there are subtle trends that can be observed. For instance, sul-fate tends to red shift the vibrational bands less significantly than other oxyanions, and selenate exhibits the largest range of vibrational frequencies (865–788 cm⁻¹).

Fig. 3. — Box and whisker plot that describes the median (solid vertical line within box), range (whiskers), and mean (x) values of symmetric stretching frequencies for uranyl mineral phases and inorganic compounds with extended topologies. Values are from the literature as summarized in Table 2.

Similarities within structural topology can also be used to understand trends in uranyl vibrational frequencies. Uranyl phosphates, which are a well-studied group of compounds due to their relative abundance as a secondary mineral phase in geologic systems and waste products [103,104], contain the two major 2-D structural topologies autunite or phosphoruranylite sheets [12]. The autunite sheet contains uranyl square bipyramids that share vertices to phosphate tetrahedral and account for 17+ different compounds. Within these compounds, the autunite sheet is retained while the identity of the charge balancing cations varies [105,106]. Phosphoruranylite sheets, which are slightly more complex, contain U(VI) with different coordination environments (square, pentagonal, and hexagonal bipyramids) leading to chain structures linked into 2-D sheets through phosphate tetrahedral [107]. This topology accounts for 11 different compounds with variability in the interstitial charge balancing cations and phosphate tetrahedra orientation. While these two classes of uranyl phosphates contain similar chemical components, both their structural complexity and vibrational spectra are unique.

A first look at the Raman spectra for the autunite family of minerals reveals a major band between 837 and 825 cm⁻¹ that was previously identified as the uranyl moiety [108–110]. Ranking the vibrational band centroid (cm⁻¹) by cation identity from largest to smallest reveals the trend Mg(II) > Ca(II) > Al(III) > Cu(II). Further investigation of the interactions between these interstitial cations and the uranyl oxo atoms can provide additional insight in the relative shift in the ν₁ band (Fig. 4). For instance, Mg(II) cations interact more with interstitial water molecules present in the interlayer region of saleeite than with uranyl unit because the distances between the uranyl oxo and the Mg cation is 4.12 and 4.14 Å (Fig. 4b) [111]. The large interatomic distance suggests no signifi-cant interaction with the uranyl bond. Replacing Mg(II) with a larger cation (Ca(II) in autunite) leads to a shorter interatomic distance (3.28 Å) between divalent cations and the uranyl oxo atoms. The uranyl symmetric stretch for autunite (830 cm⁻¹) is red-shifted from that observed in saleeite (837 cm⁻¹), suggesting a longer, weaker uranyl bond (Fig. 4c). As the sheet topology is identical between autunite and saleeite, the additional interaction between the interstitial cation likely influences the uranyl bond. Structural characterization of sabugalite has not been reported, but the spectral data suggests that Al(III) exhibits increased electrostatic interactions with the uranyl oxo atoms and a longer uranyl bond length. Inclusion of Cu(II) cations (torbernite) is unique because the d¹⁰ transition metal can form 4+2 Jahn-Teller distortions that often involve uranyl oxo groups. Thus, the distance between the uranyl oxo and Cu(II) is shorter (2.55 Å) than with other cations, and the ν₁ vibration mode red-shifts (825 cm⁻¹) accordingly (Fig. 4d) [112].

Fig. 4. — a) The autunite sheet topology contains uranyl square bipyramids (yellow polyhedra) connected to phosphate tetrahedra (green polyhedra) through shared vertices. The cation to uranyl oxo atom distance associated with (b) saleeite (Mg(II)), (c) autunite (Ca(II)), and (d) torbernite (Cu(II)) are related to the trends in the uranyl symmetric stretching mode within the Raman spectra.

A closer evaluation of the vibrational features and employment of fitting techniques reveals multiple bands in uranyl phosphate minerals that cannot be linked to crystallographically unique U atoms within the lattice. Furthermore, spectral lineshapes varied significantly when evaluating the Raman spectra. To highlight these differences, we obtained raw spectral data for meta-autunite and phosphoruranylite from the RRUFF project [113] and utilized Origin software to fit the ν₁ spectral bands. Two and three independently collected spectra were collected for phosphuranylite and autunite, respectively (Fig. 5). Striking similarities and differences are noted for each compound upon normalizing these spectra to the band at ~840 cm⁻¹ to correct for differences in detector efficiency, laser powers, and excitation geometries. First, meta-autunite contains one crystallographically unique U atom, but spectral analysis identifies at least two bands within the uranyl spectral envelope [105,114]. In addition, significant spectral variation is observed in three spectra evaluated from the database.

Fig. 5. — Comparison of solid-state Raman spectra for (A) phosphuranylite (solid black – R130108, dashed blue – R110155) and (B) autunite (solid black – R050612, dashed blue – R060434, dotted green – R060476). Spectral intensities were normalized to the highest energy bands near 840 cm⁻¹.

By comparison, phosphuranylite spectra are much more similar. First, the spectra for phosphuranylite possess at least two unique vibrational features – a primary mode centered at ~840 cm⁻¹ and a shoulder at ~820 cm⁻¹. Slight differences are noticed when making spectrum to spectrum comparison. These are highlighted in Fig. 6 where previously discussed second derivative spectral analysis was performed. In both spectra, three significant vibrational features are identified and are centered at 844/839, 820/815, and 775 cm⁻¹. This compound possesses three crystallo-graphically unique U atoms with distinct coordination geometries, and the spectral fitting also finds three bands within the complexes [107]. While clear sample to sample variations are evident, difficulties also arise from lack of spectral analysis so the ν₁ uranyl symmetric stretch for uranyl solid-state materials can be identified. This is highlighted again by the phosphuranylite system, where Driscoll et al. [109] found the major vibrational band associated with the v₁ stretching vibration to be centered at 801 cm⁻¹, whereas Frost et al. [115] and Faulques et al. [108] reported bands from 845 to 768 or at 827 cm⁻¹, respectively. Driscoll et al. [109] and Faulques et al. [108] did not attempt to fit the bands associated with the uranyl symmetric stretch, but Frost [115] identified four bands within the spectral envelop of interest and the bands exhibited variable peak widths (6–49 cm⁻¹). Moving forward, additional investigations are necessary to identify the nature and variability of the bands that arise from solid-state uranyl compounds.

Fig. 6. — Analysis of phosphuranylite spectral data in the form of Raman (A) – second derivative spectra with barcodes and (B) spectra (+ fitting) for (1) R110155 (blue) and (2) R130108 (black). Data are offset for clarity.

Vibrational analysis provides additional information regarding the unusual coordination environment of uranium within inorganic or mineral phases. In rare cases, a uranyl oxo atom can bond to a neighboring uranyl through the equatorial plane [116]. Historically, this is described as a cation-cation interaction and was previously shown to perturb the uranyl bond [116,117]. Cation-cation interactions cause the uranyl bond engaged in this interaction to weaken and the ν₁ band to red shift. Xiao et al. [118] reported Raman spectra of the two uranyl tungstate compounds, Cs₄[(UO₂)₄(WO₅)W₂O₈)O₂] and Cs₄[(UO₂)₇(WO₅)₃O₃]. These compounds contain cation-cation interactions in four of the seven crystallographically unique U(VI) polyhedra. Substantial elongation of the uranyl bonds (1.805–1.821 Å) is observed with Cs₄[(UO₂)₄(WO₅)W₂O₈)O₂] while there is more variability (1.76–1.99 Å) for Cs₄[(UO₂)₇(WO₅)₃O₃]. Concurrently, the Raman spectra reveal two uranyl vibrational modes [118]. The first is located at 828 cm⁻¹ for unperturbed uranyl bonds and a red shifted mode between ~780 and 760 cm⁻¹ for the uranyl oxo bonds engaged in cation-cation interactions. In addition, the ν₃ mode is activated because cation-cation interactions break the D_∞h symmetry. Features are weak, but can be observed between 832 and 815 cm⁻¹.

The oxidation state of the inorganic uranium compounds and minerals can be confirmed using Raman spectroscopy. While pentavalent uranium is quite rare in natural settings, hydrothermal reactions conditions in geologic or synthetic systems can result in the formation of mixed valence compounds [119–123]. X-ray diffraction data provides information on elongated bond lengths and reduced bond valence sums that would be indicative of $U (V) O_{2}^{+}$ . This oxidation state change is confirmed using Raman spectroscopy via a reduced ν₁ vibrational mode energy. The presence of U(V) within inorganic compounds and mineral phases has been suggested twice and in both instances, inconclusively. Wyartite (CaU(V)(U(VI)O₂)₂(CO₃)O₄(OH)(H₂O)₇) was characterized using Raman spectroscopy by Frost [124]. Vibrational features were observed between 850 and 830 cm⁻¹, which are consistent with U(VI), while no bands were observed in the U(V) region (800–750 cm⁻¹). In a second case, K₃(U₃O₆)(Si₂O₇) and Rb₃(U₃O₆) (Ge₂O₇) phases were synthesized under hydrothermal conditions [122]. The presence of U(V) was confirmed using XPS, XAS, and magnetic susceptibility; however, the U(V) spectral window was silent upon Raman spectroscopy analysis. These initially conflicting results can be understood by considering the covalency and Raman cross section of the uranyl moiety. Specifically, the U—O bonds within this compound give rise to a distorted octahedron, which potentially limits the covalency of the uranyl moiety thus decreasing the probability of observing this mode.

3.2. U(VI) coordination compounds and hybrid materials

Vibrational spectroscopy has been an important characterization tool for uranyl coordination compounds since the earliest reported spectra of uranyl salts by Conn in 1938 [62]. The evaluation of these simple uranyl coordination compounds using vibrational spectroscopy gave rise to our initial understanding of the uranyl cation and the covalent nature of these bonds. Over the last 10 years, instrument availability and improvements led to widespread use of these methods as characterization tools for more complex uranyl hybrid materials. With increasing use, large data sets for both coordination compounds and uranyl organic materials are available, which provides additional insights into the uranyl bond. In this section, we again focus on the use of Raman spectroscopy because of the difficult interpretation of the IR-active asymmetric stretching mode given the overlap with the fingerprint region of most organic molecules. Additional details can be found in Table 3.

Table 3.

Vibrational modes for uranyl coordination compounds and metal organic materials.

Compound	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	Refs.
Peroxides
[Na₄K₂(UO₂)₂(O₂)(C₂O₄)₄]·6H₂O	822		[349]
(NH₄)₃[(UO₂)₂(O₂)₃His(H₂O)₂]		890	[350]
(NH₄)₃[(UO₂)₂(O₂)₃salcy(H₂O)₂]		895	[350]
K₄[(UO₂)₄(O₂)₂(C₁₀H₁₃O₈N₂)₂(IO₃)₂]·16H₂O	790	890	[351]
LiK₃[(UO₂)₄(O₂)₂(C₁₀H₁₂O₈N₂)₂(H₂O)₂]·18H₂O	833	890	[351]
Na₁₀[(UO₂)O₂(C₃H₂O₄)]₅·20H₂O	806		[349]
NaK₁₅[(UO₂)₈(O₂)₈(C₁₀H₁₂O₁₀N₂)₂(C₂O₄)₄]·14H₂O	819	871	[351]
Li₄K₆[(UO₂)₈(O₂)₆(C₁₀H₁₂O₁₀N₂)₂(NO₃)₆]·26H₂O	817	890	[351]
LiU₂₄	813		[352]
BiU₂₄	814		[352]
PbU₂₄	814		[352]
LiU₂₄	814		[140]
LiU₂₈	809		[140]
U₂₄Fe₂₄P	803	873	[176]
Na₄[(UO₂)(S₂)₃](CH₃OH)₈			[353]
Halides and Hydroxides
CS₂UO₂Br₄	836	920	[101]
[Li(12-crown-4)]₂[UO₂Br₄]	822	913	[354]
[Na(15-crown-5)]₂[UO₂Br₄]	824	904	[354]
[K(18-crown-6)]₂[UO₂Br₄]	816	916	[354]
Rb₂[UO₂Cl₄]·2H₂O	839	907	[146]
CS₂[UO₂Cl₄]	832	922	[125]
CS₂[UO₂Cl₄]	834	922	[101]
[Me₄N]₂[UO₂Cl₄]	831	909	[125]
[Bmim]₂[UO₂Cl₄]	839	920	[126]
[Bmim]₂[UO₂Cl₄]	835	918	[126]
[Emim]₂[UO₂Cl₄]	827	908	[126]
[Emmim]₂[UO₂Cl₄]	835	916	[126]
[Bmmim]₂[UO₂Cl₄]	831	908	[126]
[PPh₄]₂UO₂Cl₄ (P1⁻)	838	919	[125]
[PPh₄]₂UO₂Cl₄ (P2₁/c)	823	904	[125]
[PPh₄]₂UO₂Cl₄·2CH₂Cl₂	833	912	[125]
[PPh₄]₂UO₂Cl₄·₂MeCN	832	911	[125]
[PPh₄]₂UO₂Cl₄·MeOH	836	914	[125]
[AsPh₄]₂UO₂Cl₄·2CH₂Cl₂	834	912	[125]
[K(18-crown-6)]2[UO₂Cl₄]2[UO₂Cl₄]	826	923	[354]
[Na(15-crown-5)]₂[UO₂Cl₄]₂[UO₂Cl₄]	838, 843	910	[354]
[Li(12-crown- 4)]₂[UO₂Cl₄]₂[UO₂Cl₄]	822	913	[354]
UO₂Cl₂((N(CH₃)₂)₃PO)₂	831	920	[355]
UO₂F₂(CH₃NH₂CO)	854	955	[356]
UO₂F₂DMA	852	941	[356]
UO₂F₂DMF	857	930	[356]
UO₂F₂DMSO	851	938	[356]
UO₂F₂((H₂N)₂CO)	834	915	[356]
UO₂F₂(H₂O)₂	870	940	[356]
UO₂F₂(C₁₁H₁₂N₂O)	848	935	[356]
UO₂F₂)(CH₃NH)₂CO	851	940	[356]
UO₂F₂((CH₃)₂N)₂CO	848	930	[356]
UO₂F₂((N(CH₃)₂)₃PO)₂	831	933	[355]
[Co(NH₃)₆]₂[UO₂(OH)₄]₃·H₂O	796		[357]
Nitrates
UO₂(NO₃)₂·6H₂O	871	939	[101]
UO₂(NO₃)₂·6H₂O	865		[58]
UO₂(NO₃)₂·2H₂O	876		[57]
UO₂(NO₃)₂·2H₂O	876		[57]
KUO₂(NO₃)₃	870	950	[101]
KUO₂(NO₃)₃	874		[127]
CsUO₂(NO₃)₃	874		[127]
CsUO₂(NO₃)₃	879	956	[101]
RbUO₂(NO₃)₃	880		[127]
RbUO₂(NO₃)₃	879	956	[101]
NH₄UO₂(NO₃)₃	880		[127]
	880		[128]
	880	952	[101]
Et₄N(UO₂)(NO₃)₃	871	936	[101]
(C₃H₅N₂)₂[(UO₂)₂ (μ-OH)₂(NO₃)₄]	853		[58]
[Bmim]₂[(UO₂)₂(μ-OH)₂(NO₃)₄]	839	922	[126]
(CH₃NH₂CO)₂UO₂(NO₃)₂	860	938	[101]
(phen)(UO₂)(NO₃)₂	862	942	[101]
(NO)UO₂(NO₃)₃	869	949	[358]
(Ph₃PO)₂UO₂ (NO₃)₂	848	920, 932	[101]
(Ph₃AsO)₂UO₂(NO₃)₂	818	920, 930	[101]
((EtO)₃PO)₂UO₂(NO₃)₂	860	935	[101]
[UO₂(NO₃)₂(TEM_A)₂][OTf]2	841	919	[130]
[UO₂(NO₃)₂(TEGA)₂][OTf]₂	860	932	[130]
[UO₂(NO₃)₂(TEAA)₂][OTf]₂	857	929	[130]
[UO₂(NO₃)₂(TEPA)₂][OTf]₂	854	929	[130]
[UO₂(NO₃)₂(TESUA)₂][OTf]₂	846	923	[130]
UO₂(NO₃)₂(NBP)₂	856	934	[359]
UO₂(NO₃)₂(NCMeP)₂	856	933	[359]
UO₂(NO₃)₂(DMI)₂	850	931	[359]
UO₂(NO₃)₂(N(CH₃)₂)₃PO)₂	845	932	[355]
Isothiocyanate
(C₁₀H₁₀N)₃[UO₂(NCS)₅]·3H₂O	849	906	[360]
(C₁₀H₁₀N)₃[UO₂(NCS)₅]	849.5	908	[360]
(C₁₀H₉N₂)₃[UO₂(NCS)₅]	836	897	[360]
(C₁₀H₁₀N₂)_1.5[UO₂(NCS)₅]·2H₂O	837.5	908	[360]
(C₁₀H₁₀N₂)₂[UO₂(NCS)₅]·NO₃	844	901,914	[360]
[Bmim]₃[UO₂(NCS)₅]	847	920	[126]
[(CH₃)₄N]₃UO₂(NCS)₅	841		[361]
[(C₂H₅)₄N]₃UO₂(NCS)₅	847		[361]
[(C₃H₇)₄N]₃UO₂(NCS)₅	846		[361]
(C₁₀H₁₀N₂)₂[UO₂(NCS)₄(NCS)_0.75(Cl)_0.25]·(SCN)	842	901	[360]
(C₁₀H₁₀N₂)₂[UO₂(NCS)₄Cl]·Cl·2H₂O	842	908	[360]
UO₂(NCS)₂((CH₃NH₂CO))₃	854	939	[356]
UO₂(NCS)₂((H₂N)₂CO)₃	837	910	[356]
UO₂(NCS)₂((N(CH₃)₂)₃PO)₂	840	928	[355]
[nBu4N]₃[UO₂(NCS)₅]	850	919	[362]
[Me₃NBz]₃[UO₂(NCS)₅]	845	916	[362]
[Et3NBz]3[UO₂(NCS)₅]	842	910	[362]
[Ph₄P]₂[UO₂(NCS)₃(NO₃)]	860	928	[362]
[Me₄N]₃[UO₂(NCSe)₅]·H₂O	848	920	[362]
[n Pr4N]₃[UO₂(NCSe)₅]	852	926	[362]
[Et₃NBz]₃[UO₂(NCSe)₅]	842	920	[362]
Oxygen Donors (Carboxylates, Oxalate, Ketones)
Coordination Complexes
[HNEt₃]₂[UO₂PDC₂]·2H₂O	825		[363,364]
[C₄H₁₂N₂][(UO₂)₂(C₆H₅O₇)₂](H₂O)₆	825	917	[16]
[Na₄(C₄H₁₂N₂)₆][(UO₂)₆O₂(C₆H₄O₇)₆](H₂O)₃₈	800	890	[16]
[Na₉(Mg(H₂O)₄)₃][(UO₂)₉(OH)₃O₃(C₆H₄O₇)₆](H₂O)₁₃	790	880	[16]
(Na₃[(UO₂)₃(OH)₃(ida)₃]·8H₂O	816	890	[133]
(pip)₂[(UO₂)₃O(mal)₃]·6H₂O	816	890	[133]
(pip)₆[(UO₂)₁₁(O)₄(OH)₄(mal)₆(CO₃)₂]·23H₂O	816	890	[133]
Na[Mg(H₂O)₆)][UO₂(CH₃COO)₃]₂	852	929	[365]
Na[Co(H₂O)₆)][UO₂(CH₃COO)₃]₂	851	927	[365]
Na[Ni(H₂O)₆)][UO₂(CH₃COO)₃]₂	851	928	[365]
Na[zn(H₂O)₆)][UO₂(CH₃COO)₃]₂	851	928	[365]
[UO₂(C₁₀H₈N₂)(C₇H₃Cl₂O₂)₂]	850	928	[366]
[UO₂(OH)(C₁₅H₁₁N₃)(C₇H₃Cl₂O₂)₃]	836	930	[366]
[UO₂(OH)(C₁₅H₁₀N₃Cl)(C₇H₃Cl₂O₂)₃]	841	926	[366]
[(UO₂)(NO₃)₂(HINT)₂]	828		[367]
[C₄H₁₂N₂]₂[(UO₂)(C₅O₅)₃(H₂O)]·3H₂O	848		[368]
[C₄H₁₂N₂]₂[(UO₂)(C₅O₅)₃(H₂O)]·3H₂O	833		[368]
[dabcoH₂][UO₂(C₂O₄)₂(H₂O)]·2H₂O	834	912	[369]
[dabcoH₂][(UO₂)₂(C₂O₄)₃(H₂O)₂]·2H₂O	848	928	[369]
[pipH₂] [UO₂(C₂O₄)₂(H₂O)]·4H₂O	839	924	[369]
[pipH₂][(UO₂)₂(C₂O₄)₃(H₂O)₂]·2H₂O	852	939	[369]
(UO₂)₄(pdzdc)₃(OH)₂(H₂O)₅·11H₂O	830, 845, 854, 865	928	[370]
[UO₂(C₇H₃BrIO₂)₂]_n	854	946	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄ClO₂)₂]₂	839	912	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄BrO₂)₂]	834.5	908	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄IO₂)₂]	840	912	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄ClO₂)₂]	841	922	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄BrO₂)₂]	839.5	922	[371]
[UO₂(C₁₂H₈N₂)(C₇H₄IO₂)₂	838	920	[371]
[UO₂(C₁₅H₁₁N₃)(C₇H₄ClO₂)₂]	826	916	[371]
[UO₂(C₁₅H₁₁N₃)(C₇H₄BrO₂)₂]	826	916	[371]
[UO₂(C₁₅H₁₁N₃)(C₇H₄IO₂)₂]	821.5	910	[371]
[UO₂(C₁₂H₈N₂)₂(C₇H₂F₃O₂)₂]·(C₁₂H₈N₂)	816	900	[372]
[UO₂(OH)(C₁₂H₈N₂)(C₇H₂F₃O₂)]₂	843.5	918.5	[372]
[UO₂(C₁₂H₈N₂)₂(C₇H₂Cl₃O₂)₂]·2H₂O	844	888.5	[372]
[UO₂(C₁₂H₈N₂)(C₇H₂Cl₃O₂)₂]	847	924	[372]
[UO₂(C₁₂H₈N₂)₂(C₇H₂Br₃O₂)₂]	839	886.5	[372]
[UO₂(OH)(C₁₂H₈N₂)(C₇H₂Br₃O₂)]₂	841	909	[372]
[UO₂(C₁₂H₈N₂)(C₇H₂Br₃O₂)₂]₂	874.5	937	[372]
[UO₂(C₁₂H₈N₂)(C₇H₅O₂)₂]	833	913	[372]
[UO₂(C₁₅H₁₁N₃)(C₇H₃Cl₂O₂)₂]	833	919	[373]
[UO₂(C₁₅H₁₁N₃)(C₇H₃Br₂O₂)₂]	834	920	[373]
[UO₂(C₁₅H₁₀ClN₃)(C₇H₃Cl₂O₂)₂]	836	921	[373]
[UO₂(C₁₅H10ClN₃)(C₇H₃Br₂O₂)₂]	826	910	[373]
UO₂(ba)₂ TPPO	830	938	[374]
UO₂(ba)₂ ADA	816	941	[374]
UO₂(ba)₂ AzA	814	941	[374]
UO₂(sal)₂	834	924	[375]
UO₂(2H1N)₂	841	920	[376]
UO₂(HBDM)₂	836		[377]
UO₂(HBTF)₂	830		[377]
UO₂(acac)₂	836	921	[378,379]
K₂(UO₂)(H₂O)(C₅O₅)₂	831	895	[380]
UO₂(SO₄)(HMPA)₂	834	927	[355]
α-UO₂SO₄(CO(NH₂)₂)₂	845		[290]
β-UO₂SO₄(CO(NH₂)₂)₂	859		[290]
UO₂Cl₂(THF)₂]₂	835		[381]
UO₂(Otf)₂(THF)₃	842		[381]
Coordination Polymers
Ni(UO₂)(PDC)₂(H₂O)₄]·4(H₂O)	859		[364]
Co(UO₂)(PDC)₂(H₂O)₄]·4(H₂O)	864		[364]
Fe(UO₂)(PDC)₂(H₂O)₄]·4(H₂O)	857		[364]
Zn(UO₂)(PDC)₂(H₂O)₄]·4(H₂O)	858		[364]
Co(UO₂)(PDC)₂(H₂O)₄]·4(H₂O)	860		[364]
[C₄H₁₂N₂][(UO₂)(C₄O₄)₂(H₂O)]·H₂O	829		[368]
[C₅H₆N][(UO₂)(C₄O₄)(OH)]·(H₂O)₂	834		[368]
[C₂H₁₀N₂]₂[(UO₂)₆(C₄O₄)₃(O)₂(OH)₆]	847		[368]
[C₂H₁₀N₂]₅[(UO₂)₆(C₅O₅)₆(O)₂(OH)₆(H₂O)₄]	829		[368]
(UO₂)₃(C₂H₅NO₂)₂(O)₂(OH)₂](H₂O)₆	819	904	[147]
[(UO₂)₃(C₂H₅NO₂)₂(O)₂(OH)₂](H₂O)_1.5	809	903	[147]
[(UO₂)₃(C₃H₆NO₂)₂O(OH)₃](NO₃)(H₂O)₃	840	930	[147]
[C₆H₁₃N₂][(UO₂)₃(HCOO)₂O(OH)₃]4H₂O	844	925	[147]
(C₄H₁₂N₂)[(UO₂)₂(C₄H₃O₅)₂]·4H₂O	826		[257]
[(UO₂)(C₄H₃O₅)Cu(C₁₀H₈N₂)Cl(H₂O)]·2H₂O	841		[257]
[(UO₂)₂(C₄H₃O₅)₂Cu(C₅H₅N)₂(H₂O)₂]·2H₂O	833		[257]
[(UO₂)₂(C₇O₃H₆)₂(C₇O₃H₅)(DMF)(H₂O)₃]·4H₂O	854		[136]
[(UO₂)(C₈O₄H₆)(DMF)]	841		[136]
[(UO₂)(OH)(INT)]	833		[367]
[(UO₂)₃CuO₂(C₆NO₂)₅]	809, 839	909	[382]
[(UO₂)Cu(C₆NO₂)₃]	840, 865	909	[382]
(UO₂)(C₁₄O₄H₈)	848	932	[383]
[(UO₂)₂(C₁₄O₄H₈)₂(OH)]·(NH₄)(H₂O)	873?		[383]
(UO₂)₂(C₁₄O₄H₈)(OH)₂	829, 855		[383]
(UO₂)₂[UO₄(trz)₂](OH)₂	837	904	[384]
(UO₂)₂[UO₄(trz)₂](OH)₂	769	880
(UO₂)₈O₂(OH)₄(H₂O)₄(1,3-bdc)₄ 3 4H₂O	810	880	[385]
	834	901
	846	923
	852	942
(UO₂)(OH)(Pic) (HPic = picolinic acid)	845		[255]
(NH₄)[(UO₂)₃(O)₂(OH)(Pic)₂]	820		[255]
Sr_1.5[(UO₂)₁₂(O)₃(OH)₁₃(bdc)₄]·6H₂O	851, 837, 816?		[135]
K₃[(UO₂)₁₂(O)₃(OH)₁₃(bdc)₄]·8H₂O	855, 836, 818?		[136]
(ED)[(UO₂)(btca)]·(DMSO)·3H₂O	838, 818	945, 868	[134]
(NH₄)₂[(UO₂)₆O₂(OH)₆(btca)]·~6H₂O	851		[134]
[(UO₂)₂(H₂O)(btca)]·4H₂O		946–920, 871	[134]
Nitrogen Donor Ligands
[UO₂(dapdoH₂)Cl]₂ 2,6-diacetylpyridine dioxime (dapdoH₂)	850	901	[386]
(1,8-napththalimide dioxime)(UO)₂(NO₃)(CH₃OH)	867	935	[387]
(UO₂(ZT)·₅H₂O	859	945, 821	[388]
Non-Aqueous Systems
UO₂Cl₂(C₁₄N₄H₁₆)	813		[381]
UO₂Cl₂(C₁₆N₄H₂₁)	815		[381]
UO₂(Otf)THF(C₁₆N₄H₂₁)[OTf]	833		[381]
UO₂(Otf)₂(C₁₄N₄H₁₆)	831		[381]
[UO₂(salmnt(Et₂N)₂)(H₂O)]	826	898	[389]
[UO₂(salmnt(Et₂N)₂)(pyd)]	823	893	[389]
[UO₂(salmnt(Et₂N)₂)(DMSO)]	828	885	[389]
[UO₂(salmnt(Et₂N)₂)(DMF)]	819	899	[389]
[UO₂(salmnt(Et₂N)₂)(TPPO)]	820	888	[389]
UO₂(sal-p-phdn))	842, 829	930,915	[390]
[UO₂Cl(η₃-CH(Ph₂PNSiMe₃)₂)(THF)]	825	908	[391]
[UO₂Cl(η₃-N(Ph₂PNSiMe₃)₂)(THF)]	829	909	[391]
[UO₂Cl(η₃-N(Ph₂PNSiMe₃)₂)]₂	846	924	[391]
[UO₂(N(SiMe₃)₂)(η₃-CH(Ph₂PNSiMe₃)₂)]	823	918	[391]
[Na(THF)₂][UO₂(N(SiMe₃)₂)₃]	805	928	[392]
Phosphorus Containing Ligands
AgUO₂[CH₂(PO₃)(PO₃H)]	816, 829		[393]
[Ag₂(H₂O)_1.5][(UO₂)₂[CH₂(PO₃)₂]F₂]·(H₂O)_0.5	822		[393]
Ag₂UO₂[CH₂(PO₃)₂]	802		[393]
K₅(UO₂)₂[B₂P₃O₁₂(OH)]₂(OH)(H₂O)₂	822		[394]
K₂(UO₂)₁₂B(H₂PO₄)₄](PO₄)₈(OH)(H₂O)₆	808, 839, 868		[394]
[EMim][UO₂(pmbH₂)_0.5(pmbH₂)_0.5(ox)_0.5]	831	918	[395]
[BMMim]₂[(UO₂)₂(pmbH₂)(pmb)]	824	910	[395]
[C₄mim][(UO₂)₂(1,3-pbpH)(1,3-pbpH)·Hmim]	820	909	[396]
[UO₂(1,3-pbPh₂)H₂O·mpr]	847	918	[396]
[Etpy][UO₂(1,3-pbPh₂)F]	824	908	[396]
[H₂bipy]₂[(UO₂)₆Zn₂(PO₃OH)₄(PO₄)₄]·H₂O	833		[397]
trans-UO₂(N(NO₂)₂)₂(OP(NMe₂)₃)₂	833	888	[398]
[UO₂(OP(NMe₂)₃)₄][N(NO₂)₂]₂	833	918	[398]
[UO₂(N(CN)₂)₂(OP(NMe₂)₃)₂]₂	849	928	[398]
UO₂((NC)₂C₂N₃)₂(OPPh₃)₃	854	933	[398]
[UO₂(ReO₄)₂(TPPO)₃]	823	933	[399]
[((UO₂)(TPPO)₃)₂(μ₂-O₂)][ReO₄]₂	826	931	[399]

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Average symmetric stretching mode and vibrational frequency ranges for simple coordination complexes and more complex uranyl organic compounds are provided in Fig. 7. Uranyl coordination complexes exhibit more discrete vibrational frequencies compared to the larger spectral ranges observed for complex species containing peroxides and phosphonates. This is understandable as the coordination spheres within these two types of ligands are chemically complex and change dramatically by forming diverse extended lattices. O-donating ligands such as ketones, carboxylates, and alcohols are also diverse as multiple coordination geometries and topologies can form (see Table 2).

Fig. 7. — Box and whisker plot for well-defined uranyl coordination complexes that describes the median (solid vertical line within box, range (whiskers), and mean (x) values for the uranyl symmetric stretching mode (ν₁). The X in UO₂(NO₃)₂X₂, UO₂F₂X₂ and UO₂(O₂)X_n represents substitution by a range of O- and N-donor ligands. Uranyl phosphonates and O donors (both isolated molecular species and coordination polymers with ligands containing carboxylate, oxalate, or ketones) were not included in this plot due to the diversity of coordination modes and ligands within these groups of compounds. Frequencies for all uranyl coordination complexes and polymers can be found in Table 2.

Several additional trends are noted within uranyl coordination compounds and hybrid materials. First, the coordination compounds (UO₂)L_x (X = 3, 4, or 5) show that the symmetric mode increases as follows: NO₃ < CH₃COO < SCN < Cl, Br < CO₃ < O₂ < OH. Nguyen Trung et al. [14] reported a similar trend for solution phase species. A notable exception is that solution phase UO₂(CH₃COO)₃ exhibits a band at 843 cm⁻¹, which is lower in energy than the soluble tetrahalides (UO₂Cl₄ (854 cm⁻¹) [14]. The opposite is true for solid phases.

Differences between the solution and solid-state spectra are likely related to crystallization effects as well as intermolecular interactions between neighboring cations with the uranyl oxo atoms. Careful evaluation of the uranyl tetrachloro system, which has been thoroughly characterized in solution and in 18 solid-state coordination compounds, provides us with an excellent platform to understand these effects. Schnaars and Wilson [125] systematically studied [UO₂Cl₄]²⁻ complex crystallized with the tetraphenylphosphonium/tetraphenylarsonium cation and various solvent molecules. The ν₁ mode was shown to be independent of solvent composition and tetraphenylarsonium cation presence. Crystal packing of the [PPh₄]₂UO₂Cl₄ compound induced the largest impact on this vibrational frequency. Two polymorphs formed with either a triclinic (P-1) cell (838 cm⁻¹) or a monoclinic form (823 cm⁻¹). Schnaars and Wilson [125] hypothesized that the 15 cm⁻¹ difference in the symmetric stretching band arose from either 1) additional hydrogen bonding to the uranyl oxo in the monoclinic form or 2) subtle differences in the UACl bond lengths that would lead to an increased electrostatic effect and uranyl bond destabilization. Qu et al. demonstrated similar effects by crystallizing the uranyl tetrachloro species with imidazolium cations [126]. An 8 cm⁻¹ difference was observed between [Emim]₂UO₂Cl₄ (Emim = 1-ethyl-3-methyl-imidazolium; 827 cm⁻¹) and [Emmim]₂UO₂Cl₄ (Emmim = 1-ethyl-2,3-dimethylimidazolium; 835 cm⁻¹). Hydrogen bonding interactions between uranyl oxo atoms and the charged imidazolium group were used to describe these vibrational frequency variations. Hydrogen bonding interactions were disrupted upon methyl group substitution at the N1 position for Emmim⁺. Consistent with Raman spectral features in general, this suggests that hydrogen bonding interactions and other intermolecular forces can influence the ν₁ frequency from solid-state samples.

Equatorial ligand composition also influences the ν₁ frequency because of variations in the electron-donating capabilities of the ligand and the coordination environment. The uranyl nitrate system, which has several different substitutions, provides an excellent system to discuss these differences. The ν₁ mode for X[(UO₂) (NO₃)₃] X = K, Cs, Rb, NH₄, Et₄N ranges from 880 to 870 cm⁻¹ with an average value of 875 cm⁻¹ [101,127,128]. Substitution of a nitrate group for two water molecules in the trans position occurs in case of uranyl nitrate hexahydrate [(UO₂)(NO₃)₂(H₂O)₂]·6 H₂O and the dihydrate form, resulting in a slight decrease in the symmetric stretching vibrational frequency (876–865 cm⁻¹) [57,58,129]. Ligands that provide sigma donation (PPh₃O or diamide ligands-Et₂N(C=O)(CH₂)_n(C=O)NEt₂, cyclic amides) form [(UO₂)(NO₃)₂(L)₂] species, which also cause the vibrational frequency to decrease in energy [130]. Ligand position likely influences the extent of vibrational energy change. For instance, when the nitrate groups of diamide ligands are in the trans position, the ν₁ frequency is observed between 860 and 854 cm⁻¹ [130]. Complexes that contain the nitrate ligands in the cis-conformation exhibit modes centered at 846 and 841 cm⁻¹. Similar trends are observed for cyclic amides where all three compounds contain nitrate ligands in the trans position and ν₁ bands for uranyl between 856 and 850 cm⁻¹ [130].

Hydrolysis of the U(VI) cation leads to the formation of larger oligomers and variations in the symmetric stretching mode. When hydrolysis occurs, olation or oxolation reactions result in the formation of bridged hydroxo or oxo groups [131]. These bridging groups provide additional electron donation to the metal centers, which further lowers the vibrational mode energy and causes the vibrational feature to broaden 20–30 cm⁻¹ [14,132]. The uranyl nitrate and citrate systems provide excellent platforms for demonstrating this. The [(UO₂)(NO₃)₂(H₂O)₂] complex exhibits a vibrational frequency between 876 and 865 cm⁻¹ and hydrolysis to the dimeric form [(UO₂)₂(OH)₂(NO₃)₄]²⁻ causing this mode to red shift to 853 cm⁻¹ in solid state spectra [58]. Similar trends in m₁ are observed for the uranyl citrate system. The 2:2 U:citrate dimer occurs at 825 cm⁻¹ in the solid state while the trimeric 3:2 and 3:3 U:citrate species are observed at 800 and 790 cm⁻¹, respectively[16].

Overlapping vibrational modes increase the difficulty of spectral interpretation for coordination compounds and hybrid materials, particularly in the presence of some organic ligands or peroxide molecules. Spectral features from chelating ligands were specifically noted for the citrate [16], malate [133], pyromellitate [134], benzenedicarboxylate [135], and hydrobenzoate [136] systems. Similarly, uranyl peroxide spectra contain overlapping modes from uranyl and peroxide stretches. The peroxide mode frequency depends on the coordination to uranyl polyhedra and ranges from 904 to 822 cm⁻¹ [137,138]. In the case of LiK₃[(UO₂)₄(O₂)₂(C₁₀H₁₂O₈N₂)₂(H₂O)₂]·18 H₂O, two partially resolved vibrational features are located near 830 cm⁻¹ [139]. Speciation assignment of these bands was achieved using both isotopic labeling and DFT calculations. These techniques were highlighted in the case of uranyl triperoxide (UO₂(O₂)₃)⁴⁻ complexes where the ν₁ mode was observed between 738 and 677 cm⁻¹ [140]. Empirical band predictions based upon bond distances do not accurately predict vibrational frequency, indicating a need for additional computational approaches. Quantum considerations that were developed by Vallet et al. [141] and utilized by Dembowski [138] for the peroxide system provided better agreement between the predicted (717 cm⁻¹) and experimental (710 cm⁻¹) values.

3.3. Spectroscopy as predictive tools for solid state bonds

Many spectroscopic investigations of solid-state materials focus on vibrational mode frequencies, but additional information is available from established semi-empirical relationships. The vibrational frequency can be calculated using Hooke’s law, and as a result, depends on bond lengths and force constants. Force constants provide an initial estimation of bond strength to understand the influence of the equatorial ligands on the uranyl bond [39]. Accumulated empirical vibrational spectra also allow for the estimation of vibrational frequency based on bond lengths and coordinating ligands. This section provides both the background for these relationships and an updated analysis based upon the larger data-set provided in Tables 2 and 3.

Empirical equations can be derived from experimentally observed vibrational frequencies, which are then used to calculate force constants [142,143]. For example, the vibrational frequency for a chemical bond can be modeled using Hooke’s law, and the relationship between vibrational frequency $({\bar{v}}_{vib})$ and force constant (k_F) can be derived as follows:

{\bar{v}}_{vib} = \frac{1}{2 π c} \sqrt{\frac{k_{F}}{μ}}

(1)

where ${\bar{v}}_{vib}$ , k_F, μ and c are the vibrational frequency (cm⁻¹), force constant (N/m), reduced mass (kg), and speed of light in a vacuum, respectively. The force constant can be represented using the following equations [101,144]:

4 π^{2} c^{2} {\bar{v}}_{1}^{2} = \frac{k_{F} + k_{12}}{m_{O}}

(2)

4 π^{2} c^{2} {\bar{v}}_{3}^{2} = (1 + \frac{2 m_{O}}{m_{U}}) \frac{k_{F} + k_{12}}{m_{O}}

(3)

4 π^{2} c^{2} {\bar{v}}_{2}^{2} = (1 + \frac{2 m_{O}}{m_{U}}) (\frac{2}{m_{O}}) (\frac{k_{δ}}{r^{2}})

(4)

where ${\bar{v}}_{1}$ is the symmetric stretching frequency, ${\bar{v}}_{3}$ is the asymmetric stretching frequency, ${\bar{v}}_{2}$ is the bending frequency, m_O is the atomic weight of oxygen, m_U is the atomic weight of uranium, k₁₂ is the stretching interaction constant (dynes/cm), k_δ is the bending force constant, and r is the uranyl bond length.

Force constants for uranyl compounds have been reported since the early 1960s and are now again gaining traction to quickly assess relative bond strength. McGlynn et al. [145] and Bartlett and Cooney [63] first reported a range of force constants for uranyl compounds and more recently, Schnaars and Wilson [125,146] utilized force constants within uranyl tetrachloride complexes to understand slight differences in bond strength. Force constants have also been used to evaluate subtle differences in intermolecular interactions within carboxylate complexes and to assess the impact of charge-assisted hydrogen bonding within uranyl glycine compounds [147]. Previously, the lowest k_f value for isolated uranyl complexes was reported for K₃UO₂F₅ at 6.03 mdyn/Å [63], although a majority of the values were observed between 6.4 and 7.5 mdyn/Å. Lower values were observed for extended compounds with the U(VI) cation in octahedral coordination. Expanding this to our current work reveals similar trends with a majority of compounds exhibiting force constants in the same region, even within non-aqueous compounds. One benefit in utilizing force constants is that these can provide relative bond strengths for a particular set of ligands. If we order the [(UO₂)L_x] X = 3, 4, 5 complex from largest to smallest force constants, which are NO₃ > CH₃COO > Cl, SCN > Br > CO₃ > OH, then trends in Raman vibrational frequencies are understood as these are directly related. A decrease in k_f by 0.18 is observed when [(UO₂)(NO₃)₂L] forms upon ligand substitution. It is important to note that the k₁₂ is always a small (−0.1 to −0.25) negative value that does not change significantly upon ligand substitution.

When k₁₂ is small with respect to k_F, the relationship between the symmetric stretching and asymmetric stretching modes is ν₁ = 0.939·ν₃ [101]. As a first approximation for predicting the symmetric mode frequency, this works reasonably well. The k₁₂ is always non-zero so additional formulas have been postulated for both solid state and solution samples. The empirical results for solid state species are ν₁ = 0.89· ν₃ + 30.8 (cm⁻¹) [148], 0.89· ν₃ + 21 (cm⁻¹) [145], or 0.912 ν₃ − 1.04 (cm⁻¹) while the empirical result in aqueous solution is ν₁ = 0.795· ν₃ + 107 (cm⁻¹) [149]. Cejka previously noted that these relationships are likely influenced by structural details (hydrogen bonding, crystalline packing) and character of the ligands on the equatorial plane thereby warranting further study on these relationships [39].

By focusing on the well-characterized [UO₂Cl₄]²⁻ system, a comparison among the three empirical results for ${UO}_{2}^{2 +}$ symmetric stretching feature predictions can be made. Using the equation from Bartlett and Cooney [63] (ν₁ = 0.89· ν₃ + 30.8 (cm⁻¹)) results in an average difference of 11.2 cm⁻¹ between the predicted and experimentally determined v₁ frequencies. In comparison, the predicted and experimental values differ by ~3.2 and ~3.1 cm⁻¹ when utilizing the McGlynn et al. [145] (ν₁ = 0.89·ν₃ + 21 (cm⁻¹)) or Bagnall and Wakerley [150] (ν₁ = 0.912·ν₃ − 1.04 (cm⁻¹)) relation ships, respectively. The largest differences were observed with the densely packed solids, Rb₂UO₂Cl₄·2H₂O and Cs₂UO₂Cl₄ using the Bagnall and Wakerley [150] and McGlynn et al. [145] equations where average differences were 11.8 and 8.7 cm⁻¹, respectively.

To investigate if more specialized empirical relationships improve predictions of the vibrational modes that are observed experimentally, least squares regression and well characterized compounds are used. First, ν₁ and ν₃ frequencies are plotted for uranyl tetrachloride compounds (Fig. 8). Data from densely pack solids and less resolved compounds with crown ether molecules are excluded from the analysis. Least-squares regression produced an identical relationship reported by McGlynn et al. [145] demonstrating that data segregation of specialized compounds does not lead to predictive improvements.

Fig. 8. — The least squares regression for (a) all uranyl tetrachloro compounds and (b) excluding densely packed solids and less resolved crown-ether compounds.

Similar analysis was performed by plotting all spectral data observed for uranyl carbonate and nitrate coordination complexes and hybrid materials. The empirical relationship established by McGlynn et al. [145] provided the most accurate predictions for the ν₁ mode with an average difference from experimental values of 7.8 cm⁻¹. Additional data analysis for the uranyl peroxide system and compounds synthesized in non-aqueous conditions reveals differences between predicted and experimentally observed values. Smaller differences could arise from crystallization effects as well as hydrogen bonding and cation-oxo interactions that perturb the uranyl bond within the solid-state compound.

If the force constant is known, then the bond length of uranyl bond (Å) can be estimated. The relationship between the bond length (r) obtained from crystallographic measurements and the force constant (mdyn/Å) is as follows: $r = 1.17 \cdot k_{F}^{- 1 / 3} + 1.08$ [143]. This relationship has been utilized extensively for mineralogical samples with variable degrees of success. For coordination compounds, the relationship tends to underestimate the bond distance by an average of 0.07 Å, which is significant for the relatively inflexible uranyl bond.

Vibrational frequencies for aqueous uranyl species can also be estimated by considering the coordinating ligands on the equatorial plane of the complex. Previously, an empirical equation: ν₁ (cm⁻¹) = −A·n + 870 cm⁻¹ was derived where A (cm⁻¹) is defined as the characteristic coefficient (related to the vibrational frequency difference before (870 cm⁻¹) and after uranyl coordination by a select ligand), and n is the number of ligands [151,152]. The characteristic coefficient (A) (cm⁻¹), depends on the bond strength between equatorially bound ligands to the U(VI) metal center and the length of uranyl bond. As a result, the uranyl symmetric stretching frequency decreases, and the characteristic coefficient (A (cm⁻¹)) for that ligand increases. The bond strength between uranyl and coordinating ligands typically depends on bond strength as follows: ${OH}^{-} > {CO}_{3}^{2 -} > C_{2} O_{4}^{2 -} > F^{-} > {SO}_{4}^{2 -}, {CH}_{3} {CO}_{2}^{-} > {Cl}^{-} > {Br}^{-}, {NO}_{3}^{-}$ [55,152–155]. This series indicates that when OH⁻ coordinates with uranyl, the vibrational frequency of uranyl $({\bar{v}}_{1})$ is lowest in energy, and complexes containing Br⁻ or ${NO}_{3}^{-}$ are nearly degenerate at 870 cm⁻¹.

4. Chemical identification and dynamic studies of soluble uranyl species $({UO}_{2}^{2 +})$

Vibrational spectroscopy is a powerful technique to monitor uranyl speciation in aqueous solutions, organic solvents, and ionic liquids. Signature bands can be observed for specific ${UO}_{2}^{2 +}$ complexes, which can be monitored to determine synthetic processes, determine equilibrium, or understand solvation of the uranyl cation. The importance of IR and Raman spectroscopies within uranyl solutions is highlighted in different chemical systems and solvents. Given the use of these techniques in chemistry, materials, and geoscience, the characterization of soluble uranyl species in aqueous solutions, organic solvents, and ionic liquids using vibrational spectroscopy are reported and discussed in the following sections. We specifically focus on U(VI) in this section because vibrational spectroscopy is not generally used for U(IV) detection due to the insoluble nature of these complexes and its weak spectral features.

4.1. Uranyl species in aqueous solutions

Relative abundance of each uranyl species in aqueous solution depends on the formation constant for the complex, kinetics of the system, concentrations of U(VI) and ligand, and solution pH. The pH affects the amount of hydroxide ion present in water and the amount of hydrolysis that the U(VI) metal cation will undergo to form oligomeric species. In aqueous solutions, the most soluble inorganic uranyl species contain carbonate, nitrate, and/or sulfate as inner coordination sphere ligands [156–159]. In addition, small organic molecules with O- or N-donors can also bind strongly to the uranyl cation and form soluble complexes in solution. The uranyl species in aqueous solution are now discussed in detail below and Table 4 summaries these vibrational details.

Table 4.

Vibrational modes and formation constants for uranyl species in aqueous solution.

Uranyl Species	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	log K^a	Refs.
Uranyl Species	ν₁ (cm⁻¹)	ν₃ (cm⁻¹)	log K^a	ν₁	ν₃
${UO}_{2} {(H_{2} O)}_{5}^{2 +}$	870	959		[149]	[149]
${UO}_{2} {({CO}_{3})}_{2}^{2 -}$	832		16.610 ± 0.090	[48,152,166,167]
${UO}_{2} {({CO}_{3})}_{3}^{4 -}$	812	885	21.840 ± 0.040	[48,152,166,167]	[168]
${UO}_{2} (O_{2}) {({CO}_{3})}_{2}^{4 -}$	769			[171]
UO₂(CH₃COO)⁺	861	949		[152,153]
UO₂(CH₃COO)₂	852, 841	939		[152,153]
${UO}_{2} {({CH}_{3} COO)}_{3}^{-}$	843, 823	924		[152,153]
${({UO}_{2})}_{2} {(C_{6} H_{5} O_{7})}_{2}^{2 -}$	825	919, 921		[188]	[186,187]
${({UO}_{2})}_{3} {(C_{6} H_{5} O_{7})}_{3}^{3 -}$	800	891		[188]	[186]
${({UO}_{2})}_{3} {(C_{6} H_{5} O_{7})}_{2}^{-}$	790	888		[188]	[186]
UO₂(C₂H₄O₃)⁻	(849)	932.8			[187]
UO₂(C₃H₅O₃)⁻	(847)	930.2			[187]
UO₂(C₄H₆O₅)⁻	(835)	916.2			[187]
(UO₂)_m(C₄H₄O₆)_n	(837)	918.4			[187]
UO₂F⁺	858	908	5.160 ± 0.060	[14]	[149]
UO₂F₂	846		8.830 ± 0.080	[14]
${UO}_{2} F_{3}^{-}$	834		10.900 ± 0.100	[14]
${UO}_{2} F_{4}^{2 -}$	822		11.840 ± 0.110	[14]
UO₂Cl⁺	866	962.5	0.170 ± 0.020	[152,172]	[149]
UO₂Cl₂	862	956	−1.100 ± 0.400	[152,172]	[149]
${UO}_{2} {Cl}_{3}^{-}$	858			[152,172]
${UO}_{2} {Cl}_{4}^{2 -}$	854			[152,172]
${UO}_{2} {Cl}_{5}^{3 -}$	850			[152,172]
UO₂Br⁺	870		0.220 ± 0.020	[14]
UO₂OH⁺	848.5		−5.25 ± 0.240	[14,132]
${({UO}_{2})}_{2} {(OH)}_{2}^{2 +}$	851/853	943	−5.620 ± 0.040	[14,132]	[164]
${({UO}_{2})}_{3} {(OH)}_{5}^{+}$	836	923	−15.550 ± 0.120	[14,132]	[164]
${UO}_{2} {(OH)}_{(2aq)}^{0}$	837		−12.150 ± 0.070	[14,132]
${UO}_{2} {(OH)}_{3}^{-}$	805.5		−20.250 ± 0.240	[14,132]
$({({UO}_{2})}_{3} {(OH)}_{8}^{2 -}$	812 ± 2		−32.400 ± 0.680	[14,132]
${(UO)}_{3} {(OH)}_{10}^{4 -}$	800 ± 2			[14,132]
${({UO}_{2})}_{3} {(OH)}_{11}^{5 -}$	791 ± 2			[14,132]
${UO}_{2} {(OH)}_{4}^{2 -}$	782 ± 2			[14,132]
UO₂(NO₃)⁺	870	961	0.300 ± 0.680	[152]	[149]
${UO}_{2} {ClO}_{4}^{+}$	870	962.5		[152,162]	[149]
$\equiv {SiO}_{2} {-UO}_{2} {PO}_{4}^{3 -}$	840, (837)	919		[231]	4
${UO}_{2} {SO}_{4}^{0}$	961	956	3.150 ± 0.020	[152,181,182]	[149,183]
${UO}_{2} {({SO}_{4})}_{2}^{2 -}$	852	950	4.140 ± 0.070	[152,181,182]	[183]
${UO}_{2} {({SO}_{4})}_{3}^{4 -}$	841	945	3.020 ± 0.380	[152,181,182]	[183]

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From Ref. [248].

The solvated uranyl molecule is the simplest complex observed in aqueous solutions and can be observed in the presence of weakly binding anions such as nitrate and perchlorate. Both X-ray scattering experiments and computational studies have determined that the pentaaqua complex is the dominate species under these conditions. The symmetric and asymmetric bands for the [(UO₂)(H₂O)₅]²⁺ complex is located at 870 and 962 cm⁻¹, respectively. These values generally serve as a baseline to indicate the stabilization or destabilization of the uranyl bond upon coordination to other ligands in the equatorial plane.

As indicated above, the influence of nitrate and perchlorate anions in uranyl solutions is minimal, but there is evidence that under high anion concentration in solution, complexes can form, which impact vibrational band width. Formation of the [UO₂(NO₃)]⁺ complex results in a simple spectra with a band centered at ~870 cm⁻¹ [160]. When the nitrate to uranyl ratio increases, the formation of nitrate species causes the valence electron from ${NO}_{3}^{2 -}$ to shift to ${UO}_{2}^{2 +}$ . This causes the width of the vibrational band to increase, but the position remains constant [161]. Similarly, the vibrational frequency of uranyl perchlorate is degenerate with the uranyl pentahydrate species; thus the vibrational frequency of the ν₁ band is observed at 870 cm⁻¹. These bands also exhibit similar full width at half maxima (FWHM, Γ). These spectral similarities are reasonable given that the perchlorate ligands in this system do not undergo inner sphere coordination with the uranyl cation in solution [162].

In the presence of weakly complexing ligands, hydrolysis of the uranyl cation is also commonly observed in aqueous solutions, and molecular speciation depends heavily on pH. Olation and oxolation reactions result in the formation monomers, dimers, and trimers, which increase the difficulty of spectral interpretation [131]. Consequently, knowing accurate vibrational frequencies of these species facilitates species identification. From pH 2.88–3.46, uranyl dimers, [(UO₂)₂(OH)₂]²⁺, exhibit characteristic symmetric stretching frequencies centered at 853–851 cm⁻¹ [56,163], and the asymmetric stretch appears in IR spectra at 943 cm⁻¹ [164]. At pH 4, the symmetric and asymmetric stretching modes of the trimer uranyl species, ${({UO}_{2})}_{3} {(OH)}_{5}^{+}$ , are centered at 836 [56] and 923 cm⁻¹, respectively [164]. Finally, other uranyl hydrolysis species including ${({UO}_{2})}_{3} {(OH)}_{8}^{2 -}$ , ${(UO)}_{3} {(OH)}_{10}^{4 -}$ , ${({UO}_{2})}_{3} {(OH)}_{11}^{5 -}$ , and ${UO}_{2} {(OH)}_{4}^{2 -}$ [151] are detected in solution with pH ranging from 5.63 to 14.96 [132]. The long-term stability of these hydrolysis products in aqueous solutions depends on uranyl concentration and the presence of other complexing ligands in solution.

Carbonate species are naturally abundant complexing ligands that can impart control over the presence of hydrolysis products in solution. Soluble uranyl carbonate complexes are important in environmental systems because of the equilibrium between carbonate anions and dissolved carbon dioxide in natural waters as well as the presence of widespread carbonate minerals and rock formations [165]. The two most soluble forms of carbonate species are ${UO}_{2} {({CO}_{3})}_{2}^{2 -}$ and ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ , which readily occur at neutral to basic pH values [13]. Their symmetric stretching vibrational frequencies are located at 832 and 812 cm⁻¹, respectively [14,152,166,167]. In addition, the IR-active asymmetric stretching vibrational frequencies for ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ is centered at 885 cm⁻¹ [168]. At near neutral pH conditions, there is an equilibrium between uranyl carbonate species and hydrolysis products in solution that can easily be monitored using vibrational spectroscopy. Lu et al. [48] analyzed the spectra of aqueous solution containing ${UO}_{2 (aq)}^{2 +}$ and 105 mM Na₂CO₃ at pH 3, 6, and 11 and compared these results to the expected equilibrium diagram (Fig. 9). The solution at pH 3 was predicted to contain free ${UO}_{2}^{2 +}$ cation as well as a small amount of the ${({UO}_{2})}_{2} {(OH)}_{(aq)}^{3 +}$ phase, and this was confirmed with the bands at 871 and 859 cm⁻¹ that were observed in the Raman spectra [48]. Under alkaline conditions, the only signal observed in Raman spectra was located at 814 cm⁻¹, which corresponds to the only equilibrium diagram predicted species, ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ . The diagram also predicted the formation of a Na₂U₂O₇ solid precipitate, but this did not form until 12 days after sample preparation. Similar slow kinetics were observed under near neutral conditions, as hydrolysis products ${({UO}_{2})}_{2} {(OH)}_{2}^{2 +}$ and ${({UO}_{2})}_{3} {(OH)}_{5}^{+}$ along with ${UO}_{2} {({CO}_{3})}_{2}^{2 -}$ and ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ were observed in spectra. The equilibrium diagram indicated that hydrolysis products should not be present in the solution under these conditions and indeed the spectral signals associated with the hydroxide phases were absent after 12 days of aging the sample.

Fig. 9. — Investigations by Lu et al.[48] utilized Raman spectroscopy to explore the presence of uranyl carbonate and hydroxide phases in aqueous solutions and related these values to the calculated equilibrium diagram. Slow kinetics resulted in initial differences between spectral analysis and predicted values. Reprinted with permission from Ref. [49]. Copyright 2016 American Chemical Society.

Alpha radiolysis of carbonate-rich water can result in the formation of peroxide and stable peroxocarbonate species that have importance in the corrosion of spent nuclear fuel [169,170]. Peroxide forms in aqueous solution due to the alpha-radiolysis of water and precipitates to the previously described studtite phase at lowpH conditions. In higher pH solutions, mixed peroxocarbonato species [UO₂(O₂)_x(CO₃)_y]^2−2x−2y forms with x:y = 1:2, 2:0, and 0:3 complexes present depending on the solution pH values [171]. Raman spectroscopy can be used to differential these complexes as the symmetric stretching bands for $({UO}_{2} (O_{2}) {({CO}_{3})}_{2}^{4 -})$ , $({UO}_{2} {({CO}_{3})}_{3}^{4 -})$ , and $({UO}_{2} {(O_{2})}_{2}^{2 -})$ are centered at 769, 811, and 848 cm⁻¹, respectively [171]. The uranyl peroxocarbonato complex is not stable in solution for long periods of time and decomposes to the uranyl tricarbonato species under alkaline conditions or elevated temperatures (Fig. 10). Kim et al. monitored this decomposition reaction using Raman spectroscopy and determined the activation energy of the reaction to be 7.144 × 10³ J/mol [169].

Fig. 10. — Raman spectra of (A) uranyl in 0.5 M Na₂CO₃ with 1.0 M H₂O₂ at pH 11.4, and (B) uranyl in 0.5 M Na₂CO₃ at pH 10.8. Vibrational frequency of uranyl species: (1) 848 $({UO}_{2} {(O_{2})}_{2}^{2 -})$ , (2) 811 $({UO}_{2} {({CO}_{3})}_{3}^{4 -})$ , (3) 769 $({UO}_{2} (O_{2}) {({CO}_{3})}_{2}^{4 -})$ , and (4) 727 $({UO}_{2} {({CO}_{3})}_{x} {(OH)}_{y}^{2 - 2 x - 2 y})$ cm⁻¹. Figure is reprinted with permission from Ref. [171]. Copyright 2012 American Chemical Society.

Uranyl halide complexes form in high ionic strength salt solutions, which is hypothesized to impact the uranyl symmetric stretching mode energy via electrostatic interactions between the U(VI) cation and halide anions. Monomeric UO₂X_n ²⁻ⁿ (X = F⁻, Cl⁻, and Br⁻; n = 1, 2, 3, 4) complexes have been previously characterized in aqueous solutions by Nyugen-Trung et al. [14]. The conditions needed to form these complexes vary, and the UO₂F⁺, UO₂Cl⁺, and UO₂Br⁺ species form when the ligand to U(VI) ratio is 1, 300, and 500, respectively. The impact of halide ligands on the uranyl symmetric stretching mode decreases by 12, 4, and 0 cm⁻¹ (from the ${UO}_{2} {(H_{2} O)}_{5}^{2 +}$ complex) when the uranyl cation is coordinated by four fluoride, chloride, and bromide anions, respectively [152,172]. Vallet et al. utilized quantum chemical methods to probe the nature of the uranyl halide interaction and concluded that electrostatics alone could sufficiently cause bond destabilization, which is influenced by ligand electronegativity [141]. This result corresponds to spectroscopic trends where the more electronegative F⁻ anion influences the band position more than the Cl⁻ anion.

Pure uranyl peroxide species can also be observed in solution and exhibit complex speciation with the formation of large, fullerene-type nanoclusters under alkaline conditions [173]. While monomeric ( $({UO}_{2}) {(H_{2} O)}_{2} {(O_{2})}_{2}^{2 -}$ , $({UO}_{2}) (H_{2} O) (OOH) {(O_{2})}_{2}^{3 -}$ , and $({UO}_{2}) (OOH) {(O_{2})}_{2}^{3 -}$ ) as well as dimeric $({({UO}_{2})}_{2} (μ^{2} - O_{2}) {(O_{2})}_{4}^{6 -})$ uranyl peroxide complexes are expected to be present in basic systems, larger nanoclusters containing 24–60 uranyl peroxide units have also be identified as stable species in aqueous solutions [137,174]. Raman spectroscopy has been used as a tool to explore the presence of these uranyl peroxide species in solution, and the signals identified in the spectra include bands between 900 and 750 cm⁻¹. The peroxide symmetric stretching mode has been identified in the range between 850 and 820 cm⁻¹ and the band located between 820 and 750 cm⁻¹ has been previously assigned to the symmetric stretching mode for the uranyl cation for solutions containing U₂₄ and U₆₀ clusters at concentrations of 45 or 85 mg/mL [137]. In addition, a third band can be observed in solution at 878 cm⁻¹ that is absent in the solid state spectra. McGrail et al. postulated that this band could be associated with the peroxo stretching mode when there are a limited number of coordinating counter cations present in the solution [137].

Larger uranyl peroxide nanoclusters can also be formed in the presence of oxyanions with phosphate, pyrophosphate, and phosphonate anions yielding a maximum number of characterized clusters [175–178]. The stability of the U₂₀P₆ cluster, which contains phosphate anions, was evaluated using NMR, SAXS, and Raman spectroscopy [178]. The solid state crystalline material was redis-solved in aqueous solution, and the original solution revealed vibrational bands at 805 and 836 cm⁻¹ that were assigned to the uranyl symmetric stretch and the peroxide stretch, respectively. These bands blue-shifted to 808 and 847 cm⁻¹ after aging the solution for 48 h, which suggested that the original U₂₀P₆ cluster transformed to the widely observed U₂₄ species [178].

The uranyl cation forms complexes with a wide range of oxyanions in aqueous solution, but only a handful of these have been systematically evaluated in solution. Some anions, particularly phosphate and vanadate, form solid compounds with low solubilities (10⁻⁷ to 10⁻⁹ M for uranyl phosphate and vanadates) [20,179,180]; thus, low concentrations in solution limit spectral characterization. Silicate forms a rather weak uranyl complex, and ${UO}_{2} (SiO {(OH)}_{3}^{+}$ is the only major species observed between the narrow pH regime from 5 to 6.5. Due to the limited range of this uranyl silicate species, there has not been spectral characterization reported in the literature. Uranyl sulfate species including UO₂SO₄, ${UO}_{2} {({SO}_{4})}_{2}^{2 -}$ , and ${UO}_{2} {({SO}_{4})}_{3}^{4 -}$ are soluble and have been identified at 861, 852, and 841 cm⁻¹ in Raman spectra [14,152,181,182], and 956, 950, and 945 cm⁻¹ in IR spectra, respectively [183]. Ligand binding in the uranyl sulfate complexes vary as UO₂SO₄ is considered in the mono-dentate structure, while ${UO}_{2} {({SO}_{4})}_{2}^{2 -}$ is bidentate as indicated from EXAFS measurements [184]. Differences in chelation can also result in changes within active modes of the sulfate.

Carboxylate ligands including acetate [152,185], citrate [186], glycolate [187], lactate [187], malate [187], oxalate [152,188], succinate [185], tartrate [187] and tricarboxylate [187] ions, readily form complexes with the uranyl cation and are important species in natural and biological systems. Hexavalent uranium is a hard Lewis acid that prefers hard O-donors; thus, carboxylate complexes are stable in water and have formation constants (log K) that range between 2 and 16 [189–192]. The overall coordination geometry of these species were previously studied using density functional theory (DFT) calculations, which predicted that both mono- and bi-dentate coordination structures are possible but depend on ligand stereochemistry [185]. Organic acids contain a hydroxyl group adjacent to a carbonyl carbon, such as in the case of citrate $(C_{6} H_{5} O_{7}^{3 -})$ , glycolate $(C_{2} H_{4} O_{3}^{-})$ , lactate $(C_{3} H_{5} O_{3}^{-})$ , malate $(C_{4} H_{6} O_{5}^{-})$ , and tartrate $(C_{4} H_{4} O_{6}^{2 -})$ , coordinate with uranyl via one oxygen from both the carboxyl and alcohol groups [187]. Uranyl complexation to these molecules is pH dependent in solution because of the need to deprotonate the carboxylate before binding can occur. This was demonstrated in the uranyl acetate system where U(VI) ligand complexes do not form at pH 2.5, even up to a ratio of acetate to uranyl of 20 because the pK_a of acetic acid is 4.75 [152]. The pK_a of the alcohol group is generally much higher so binding to the uranyl cation can occur in the protonated form. The observed asymmetric stretching IR bands and calculated symmetric stretching Raman bands (in parenthesis) [149] are 921 (840), 932.8 (849), 930.2 (847), 916.2 (835), and 918.4 (837) for uranyl citrate $(C_{6} H_{5} O_{7}^{3 -})$ , glycolate $(C_{2} H_{4} O_{3}^{-})$ , lactate $(C_{3} H_{5} O_{3}^{-})$ , malate $(C_{4} H_{6} O_{5}^{-})$ , and tartrate $(C_{4} H_{4} O_{6}^{2 -})$ , respectively [168,187].

Spectral interpretation of the uranyl carboxylate system can be complex due to the variability in chelation and activation of modes upon complexation to the U(VI) cation. Simple uranyl acetate species include UO₂(CH₃COO)⁺, UO₂(CH₃COO)₂, and ${UO}_{2} {({CH}_{3} COO)}_{3}^{-}$ with identified symmetric stretches at 861, 852, and 843 cm⁻¹ and asymmetric stretches at 949, 939, and 924 cm⁻¹, respectively [185]. Initial investigation of the vibrational frequencies of the uranyl symmetric stretches for UO₂(CH₃COO)₂ and ${UO}_{2} {({CH}_{3} COO)}_{3}^{-}$ were assigned to bands centered at 841 and 823 cm⁻¹ vs. 852 and 843 cm⁻¹, respectively [153]. This assignment discrepancy is likely attributed to overlapping vibrational bands from various uranyl species and (the lack of) band deconvolution used during spectral analysis. This also occurred for succinic acid, which also chelates to the uranyl cation through mono- or bidentation coordination modes and displays multiple vibrational frequencies in ATRIR spectra (950, 938, and 925 cm⁻¹) [185].

Assignments of uranyl citrate species were also difficult to interpret due to the presence of overlapping bands combined with the formation of stable hydrolysis products in solution. Uranyl citrate species are highly soluble and used to improve U removal from soils and enhance phytoremediation strategies [193,194]. NMR studies from Nunes et al. [195] defined several well-defined species within this system with U(VI):ligand ratios of 2:2, 3:3, and 3:2. Raman studies found multiple bands centered at 852, 812, and 797 cm⁻¹ that were initially assigned to the 2:2, 3:3, and 3:2 complexes [186]. More recent studies determined that the band at 812 cm⁻¹ was associated with the activation of a citrate vibrational mode that is only observed upon complexation to the uranyl cation. Current assignments for the uranyl symmetric stretches of ${({UO}_{2})}_{2} {(C_{6} H_{5} O_{7})}_{2}^{2 -}$ , ${({UO}_{2})}_{3} {(C_{6} H_{5} O_{7})}_{3}^{3 -}$ , (UO₂)₃(C₆H₅O₇)₂ are located at 825, 800, and 790 cm⁻¹ (Fig. 11), [16] and asymmetric stretches at 919, 891, and 888 cm⁻¹, respectively [186].

Fig. 11. — Raman spectra of aqueous solutions at pH 3, 5, 7, and 9 containing $[{UO}_{2}^{2 +}] : [C_{6} H_{5} O_{7}^{3 -}]$ in the ratio of (a) 1:1 and (b) 1:2 indicate a shift from a dominant dimeric species to a trimer with increasing pH. Peaks were normalized based upon the ${NO}_{3}^{-}$ peak at 1047 cm⁻¹, and the dashed line indicates the expected vibrational frequency from [(UO₂) (H₂O)₅]²⁺. Vibrational frequencies of uranyl citrates are (1) 825 $({({UO}_{2})}_{2} {(C_{6} H_{5} O_{7})}_{2}^{2 -})$ , (2) 800 $({({UO}_{2})}_{3} {(C_{6} H_{5} O_{7})}_{3}^{3 -})$ , and 790 ((UO₂)₃(C₆H₅O₇)₂) cm⁻¹. Reprinted from Ref. [16] with permission from The Royal Society of Chemistry.

Much of this section has focused on well-defined systems, but the speciation of U(VI) in the presence of complex organic biomolecules, like fulvic and humic acids, are vitally important for understanding the fate and transport of U in natural systems. Humic and fulvic acids represent families of organic acids that vary in acidity, polymerization, and functional groups. These organic biomolecules can mobilize natural uranium and enhance the transport within subsurface systems.[196] Vibrational spectroscopy is an important tool for developing an enhanced understanding of U(VI) speciation in these systems as demonstrated by Stuedtner et al. [197]. In this study, ATR FT-IR was paired with geochemical modeling to identify binary and ternary U(VI) humate (HA) complexes. Under N₂ gas, the uranyl cation binds with humic acid to form two soluble species (UO₂(HA) (ν₃ = 925 cm⁻¹) and (UO₂(OH)(HA)(ν₃ = 913 cm⁻¹)) and one solid phase (ν₃ = 932 cm⁻¹). In the presence of CO₂, a ternary complex (UO₂)(CO₃)₂HA (ν₃ = 892 cm⁻¹) is expected to form and dominate under alkaline conditions [197]. Additional studies are necessary to provide details on the exact speciation in the presence of organic acids and explore other biomolecules relevant for environmental transport and human health.

4.2. Uranyl species in ionic liquids

Ionic liquids have been explored as novel solvents for uranyl extraction from aqueous solution, and vibrational spectroscopy is the primary characterization tool for assessing dominant uranyl species. Pasilis et al. explored the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf₂N]) system and observed that the addition of uranyl perchlorate resulted in the formation of a symmetric stretch mode at 884 cm⁻¹, which is attributed to the hydrated uranyl perchlorate complex [198]. Addition of tetrabutylammonium nitrate to this system resulted in the displacement of water and perchlorate ligands thereby forming two uranyl nitrate species (UO₂(NO₃)₂, and ${UO}_{2} {({NO}_{3})}_{3}^{-}$ ) with Raman vibrational frequencies centered at 869 and 865 cm⁻¹, respectively. Hopkins et al. investigated the ionic liquid, aluminum chloride-1-ethyl-3-methylimidazolium chloride (AlCl₃-EMIC) and determined that the major species was the uranyl chloride complex $({UO}_{2} {Cl}_{4}^{2 -})$ [199]. The vibrational frequency of this species in the ionic liquid is located at 838 cm⁻¹, which is similar to the solid state vibrational frequency for Cs₂UO₂Cl₄ that is centered at 836 cm⁻¹ [200]. As a comparison, the vibrational frequency for ${UO}_{2} {Cl}_{4}^{2 -}$ is located at 854 cm⁻¹ in aqueous solution [152]. This suggests that the environment around the ${UO}_{2} {Cl}_{4}^{2 -}$ complex in ionic liquids may be similar to that of the densely-packed cesium uranyl chloride salt lattice. More recent studies by Krishna et al., confirmed the that the tetrachloro species is present in 1-butyl-3-methylimidazlium bis(trifuloromethylsulfonylimide (C(4)mimNTf (2)) ionic liquid and found that these complexes underwent facile and favorable electrochemical reduction [201,202].

If the ionic liquid contains strongly complexing functional groups, then additional complexes must be considered when describing the chemical complexity within these systems. For example, the addition of uranyl perchlorate in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate (BMINfO) results in the displacement of the perchlorate anions by the sulfonate group in BMINfO. This results in a red-shift of the uranyl symmetric stretch from 880 [162,203] to 871 cm⁻¹ [204]. Other ionic liquids, such as those containing dicyanamide [205], have the ability to complex with the uranyl cation as evidenced by crystallization studies. Additional experiments are necessary to determine the vibrational bands present in these solvents and the identity of other uranyl species that form in ionic liquids with O-and N-donor groups.

4.3. Uranyl species in polar organic solvents

While there is a large body of literature on the use of vibrational spectra to determine aqueous uranyl species, much less is known about speciation of uranium in polar organic solvents. One major difference between non-aqueous and aqueous solutions is that many organic solvent molecules do not inhabit the inner sphere coordination of the uranyl cation [206,207]. The most prominent differences in vibrational mode frequency are observed in diethyl ether (C₂H₅O). The vibrational frequency (ν₃) of UO₂(NO₃)₂·2(C₂H₅O)_(s) is centered at 940 cm⁻¹ in the solid phase, and [UO₂(NO₃)₂] dissolved in C₂H₅O exhibits an identical vibration frequency [207]. This same vibrational frequency is observed for [UO₂(NO₃)₂] dissolved in other organic solvents including methanol, butanol, iso-butanol, acetone, and methyl ethyl ketone [207]. As such, the inner sphere coordination for this species is unlikely. Additional studies are necessary to explore solvents with ligands that may complex via inner sphere coordination with the uranium, including pyridine, DMSO, and THF.

4.4. Using vibrational spectroscopy to provide thermodynamics of solution-phase species

As the previous sections highlight, vibrational spectroscopy is a powerful tool for developing speciation models and stability constants in uranyl systems. Analyzing either the band intensity or areas associated with the ν₁ or ν₃ band for the uranyl species present in solution provides information regarding the concentration of the various complexes. Typically, this is done by assuming that the cross section is the same across similar species. This was demonstrated vide supra with the uranyl carbonate system [48], but advanced chemometric methods can also be used for quantifying relative amounts of various species in more complicated systems. Chemometrics [155] successfully facilitated the identification and quantification of oligomeric uranyl species (U_total = 0.1 M) present in solutions with pH ranging from 1.55 to 4.20 using both Raman and FT-IR spectroscopies. The multivariate method partial least squares (PLS2) was employed and revealed the relative abundance of ${({UO}_{2})}_{2} {(OH)}_{2}^{2 +}$ and ${({UO}_{2})}_{3} {(OH)}_{5}^{+}$ in aqueous systems. Importantly, this approach eliminated the possibility that the previously reported uranyl species (UO₂)₂OH³⁺ existed under these conditions.

Linear free energy relationships can also be developed to determine stability constants for U(VI) complexes. The relationship between the overall stability constant (log β) and the shift in the Raman vibrational frequency was highlighted in the study by Nguyen-Trung et al. [14]. Plotting these values lead to straight lines that can be used to estimate the stability constant for mononuclear U(VI) complexes with the following equations:

log β_{1} (U O_{2}) L = - 0.52 Δ v_{1} - 1.61

(5)

log β_{2} (U O_{2}) L_{2} = - 0.50 Δ v_{1} - 4.10

(6)

log β_{3} (U O_{2}) L_{3} = - 0.46 Δ v_{1} - 5.68

(7)

It must be noted that a complete understanding and identification of species present in solution are necessary to effectively utilize these equations. In addition, Nguyen-Trung noted that there are significant variations in spectral interpretation and large uncertainities (as much as two log units for β) for the stability constants published in the literature [14]. This suggests the need for additional studies to further explore these relationships.

5. Surface speciation studied by IR and Raman spectroscopies

Uranium adsorption onto surfaces has implications for environmental transport modeling, remediation strategies, and radio-chemical separations that can be further understood using vibrational spectroscopy [208–211]. Both U(IV) and U(VI) readily hydrolyzes in aqueous solution, which also promotes their adsorption onto mineral surfaces [212,213]. This is important for modeling uranium transport in natural waters, but also has importance in Mn hydroxide precipitation steps during radiochemical separations protocols [214,215]. In addition, U(VI) species readily interacts with organic matter associated with soil particles [209] and adsorbs to the organic ligands present in separation columns [216]. Vibrational spectra can provide chemical insight via band frequencies and bandwidths, both of which that aid in the characterization of uranyl surface species and local environmental conditions. Much of the work involving the characterization of uranyl sorption onto surfaces has been performed with IR spectroscopy; therefore, much of this section will include the assessment of the asymmetric stretching vibrational mode.

5.1. Uranyl adsorption onto oxide surfaces

The vibrational frequencies of uranyl species adsorbed to oxide and oxyhydroxide surfaces have been rigorously reported in the literature. For instance, uranyl adsorption to hematite (α-Fe₂O₃) [214,217–219], maghemite (γ-Fe₂O₃) [220], ferrihydrite (Fe₂O₃·0.5H₂O) [221,222], smectite [223], hydrous titania (TiO₂·nH₂O) [224], hydrous zirconia (ZrO₂·nH₂O) [224], and hydrous silica gel (SiO₂·nH₂O) has been explored using vibrational spectroscopy [224]. Variations in vibrational frequency are observed from adsorbed uranyl species indicating the presence of multiple binding structures and/or species that depend on the oxide mineral identify (Fig. 12). For instance, the ν₃ asymmetric mode for uranium species adsorbed to maghemite and ferrihydrite surfaces are centered at 912 and 903 cm⁻¹, respectively [220,221]. The vibrational band of uranyl adsorbed to ferrihydrite is red-shifted compared to that to maghemite surfaces indicating stronger interactions between uranium and ferrihydrite. This is further supported as uranyl desorbs less from ferrihydrite versus maghemite [220]. Outer sphere coordination mode was postulated for uranyl on the surface of maghemite from the asymmetric stretching (v3) modes whereas a bridging bidentate inner sphere coordination is predicted for ferrihydrite. An inner sphere complex is also predicted for SiO₂ and Al₂O₃, but the Raman modes are blue-shifted from spectral features collected from iron oxide surfaces, suggesting a weak interaction with the oxide surface [224–226].

Fig. 12. — Previously reported uranyl species and asymmetric stretching frequencies (v3) for isolated aqueous and surface complexes and extended solid-state precipitates on metal (M) oxides (O). The pH for all reported studies range from 5 to 8 and the surface state is depicted as O atoms for simplicity. Data based upon Refs. [152,215,218,220,221,224–227].

Vibrational spectroscopy is a powerful technique for observing changes in uranyl speciation from environmental perturbations, including pH and carbonate concentration variations. Under acidic conditions, adsorption of uranium to a solid is generally reduced. This occurs because the positively charged major uranyl cation in solution, $U (VI) O_{2}^{2 +}$ , is repelled by the positively charged metal oxide surface [228]. Increasing pH leads to the formation of hydrolysis products, such as ${({UO}_{2})}_{3} {(OH)}_{5}^{+}$ ; but in the presence of carbonate, both ${({UO}_{2})}_{2} {CO}_{3} {(OH)}_{3}^{-}$ and ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ complexes form [8,229]. Changes in uranyl speciation subsequently and significantly influence surface adsorption energetics while surface complex identity is less impacted if adsorption occurs. For instance, the adsorption capacity of uranyl on hematite is maximized when the pH is above 6.2 [227]. The asymmetric stretching band of the uranyl cation, however, remains at 906 cm⁻¹ while the intensity increases upon adsorption to the mineral surface. In the presence of carbonate, adsorption capacity decreases in the same pH range, suggesting the formation of free ${({UO}_{2})}_{2} {CO}_{3} {(OH)}_{3}^{-}$ and ${UO}_{2} {({CO}_{3})}_{3}^{4 -}$ complexes [214]. The vibrational frequencies of uranyl adsorbed to ferrihydrite are observed at 889 cm⁻¹, suggesting the formation of ternary uranyl carbonate complexes. Vibrational spectroscopy has explored the interaction between ferrihydrite surfaces and uranyl carbonate complexes and revealed a red-shift in the asymmetric stretch from 902 to 881 cm⁻¹ [222].

5.2. Identification of insoluble aqueous species on solid-state surfaces

In some cases, examining surface chemical composition provides information on low abundance, insoluble species, whose detectability in solution is near the detection limit of vibrational spectroscopy [230]. For instance, uranyl phosphate complexes with a formula of (UO₂)₃(PO₄)₂·4H₂O form in solution with pH ranging from 1.5 to 9 [230]. These species remain stable at dilute concentrations because of their low solubility in water [179]. Experimentally, phosphate species can be investigated by detecting adsorbed species to SiO₂ crystals using ATR FT-IR spectroscopy [230]. Species detected using this method were $\equiv {SiO}_{2} - {UO}_{2} {PO}_{4}^{3 -}$ and possessed a vibrational frequency at 919 cm⁻¹, which is identical to that for (UO₂)₃(PO₄)₂·6H₂O_(s). In addition, Raman bands for the solid-phase uranyl complexes [UO₂(PO₄)]⁻ (ν₁ = 840 cm⁻¹) and [UO₂(PO₄)(H₂O)₄]⁻ (ν₁ = 841 cm⁻¹) [231] are similar to calculated vibrational frequencies, both of which are predicted to be centered at 837 cm⁻¹ [149].

5.3. Monitoring uranium redox using vibrational spectroscopy

U(IV) redox processes that occur on mineral surfaces can also be explored using vibrational spectroscopy. In some cases, the reduction of U(VI) was monitored to explore the retention of uranium on mineral surfaces. For example, Wersin et al. investigated uranyl adsorption to sulfide bearing minerals including (PbS) and pyrite (FeS₂) using Raman spectroscopy and monitored the redox promoted generation of the surface precipitate U₃O_8(s) [232]. Oxidation and corrosion of spent nuclear fuel is a significant area of study, and Raman spectroscopy is a powerful tool for identifying fuel alteration under repository conditions. Bonales et al. reported spent nuclear fuel oxidizes into U₃O₈ upon exposure to air or into uranyl carbonates and silicates if exposed to groundwater from Sierra Albarrana (Spain) [71,233]. Schmeide et al. explored the redox behavior of uranyl on silicate rock (diorite) surfaces under anoxic conditions and found that uranyl carbonate species reduced the affinity of U(VI) to mineral surfaces and stabilized soluble high valence species in solution [234]. In a final example, Traboulsi et al. used Raman spectroscopy to explore alpha radiolysis of water and oxidation of solid UO₂ to form uranyl peroxide species [66,235].

5.4. Changes in Raman bandwidths on solid state surfaces

Vibrational mode bandwidths are important components when evaluating and reporting the uranium speciation upon adsorption. Increases in bandwidth upon adsorption arise from the formation of multiple uranyl species, varying uranyl orientations, and/or activation of multiple vibrational modes all of which can overlap spectrally [214]. For example, the bandwidth (FWHM) of a ν₁ Raman mode for an adsorbed species is ~57 cm⁻¹, which is broader [224] than a variety of uranium species in solution (FWHM = 13–20 cm⁻¹) [48]. Multiple uranyl species can arise from original surface speciation or change upon adsorption with or without coordinated ligand displacement [221,224,236–238]. Inner-sphere coordination increases the vibrational mode band width to a greater extent than outer-sphere coordination. This is because the binding strength of the ligands to the uranyl cation influences the exact nature of these interaction and vibrational mode perturbations [237]. Additional broadening can be caused by bond contraction on the surface, crystallite size, temperature, pressure, and crystallinity. While some of these properties only minimally impact vibrational linewidths, it is important to note that these can induce slight variations.

5.5. Spectral signals collected on functionalized solid materials

Radiochemical separations often rely on surface-driven adsorption processes, and vibrational spectroscopy methods can be used in conjunction with other techniques to provide insight into chemical binding and resulting surface speciation. In addition to the previously discussed oxide materials, both functionalized solids including silica and polymers have been used for uranium extraction. Huynh et al. reported 250 mg U(VI) uptake per g of a sorbent composed of amine functionalized mesoporous silica (SBA-15). Both IR (v₃ = 909 cm⁻¹) and Raman (v₁ = 827 cm⁻¹) spectroscopies suggested inner sphere coordination with the surface amines of the sorbent [239]. Amidoxime-containing materials have been shown to selectively uptake U(VI). Vibrational spectroscopy has helped identify surface speciation in these systems [236]. For instance, polyacrylonitrile polymers are easily functionalized with amidoxime groups, which makes these types of materials of particular interest for extracting U(VI) from sea water. The coordination geometry of uranium adsorbed to amidoxime have been studied using extended x-ray absorption fine structure (EXAFS) [240], X-ray absorption near-edge structure (XANES) [241], density function theory (DFT) [242,243], as well as Raman and IR spectroscopy [242,244–246]. DFT studies suggested that uranyl coordinated with amidoxime groups form tridentate or η² – coordination structures [243]. This provided support with respect to the high extraction efficiency of amidoximated materials for uranyl; however, Abney et al., utilized EXAFS to explore the coordination and concluded that cooperative binding from multiple sites was a more likely binding mechanism [240]. Future studies utilizing vibrational spectroscopy could provide additional insight into the coordination of the U(VI) on amidoxime surfaces.

6. Challenges, opportunities, and vision

This review has highlighted the importance of how vibrational spectroscopy can provide insight into uranium chemistry and speciation, but there is an opportunity for the development of new methodologies and techniques that can lead to additional advancements in characterization and detection. Current limitations include deconvolution of overlapping vibrational bands in the narrow uranyl spectral window [152] and the relatively low signals from vibrational modes compared to signal collected using other methods, such as fluorescence spectroscopy [247]. Spectral deconvolution techniques can overcome the limitation of unresolved spectral features, and additional computational assessment can provide supporting information on vibrational band assignments and questions related to electronic structure of the uranium cation. Furthermore, uranyl detectability can be improved by introducing nanomaterials to enhance vibrational band intensities. As a result, the application of vibrational spectroscopy for uranium speciation analysis and detection can be widened compared to current capabilities. In the next sections, we highlight the current challenges and present opportunities for overcoming these limitations.

6.1. Uranium detection in complex aqueous matrices

Complex samples, such as those found in environmental and biological systems, can significantly hamper the analysis of vibrational spectra. Matrix effects pose challenges in that detection (signal to noise) limits are higher than in simple systems, and increased diversity of uranium species present in a sample increases the possibility that vibrational bands overlap in a spectrum. Many times, resulting spectra are analyzed by reporting the center (i.e., centroid) of a broad uranyl feature, but these broad spectral bands likely do not correspond to any one uranyl species present in a sample. One way to overcome this challenge is to determine the relative abundance of each uranyl species from spectral analysis and compare these to a model generated from known thermodynamic constants [157,248–250]. Dargent et al. evaluated solutions containing 10 mM uranyl and chloride concentrations ranging from 0.3 to 12 M using Raman spectroscopy, and the spectral features were fit using pseudo-Voigt functions (a combination of Gaussian and Lorentzian functions) to obtain the equilibrium constants [172]. Lu et al. developed a step-by-step process for Raman scattering peak analysis using Gaussian function fitting (Fig. 13). This approach facilitated the accurate extraction of uranyl speciation from uranyl solutions prepared in the presence of the commonly observed ligands including ${CO}_{3}^{2 -}$ , ${NO}_{3}^{-}$ , and ${SO}_{4}^{2 -}$ as well as a function of pH [48]. Using this approach, the spectral fitting procedure resulted in accurate relative abundance predictions for species from equilibrium diagrams and also provided speciation predictions from samples that were not at equilibrium. As a result, vibrational spectroscopy coupled with careful analysis could provide realistic details regarding the complex and dynamic speciation of uranyl in samples.

Fig. 13. — Detect and identify uranium species in aqueous solution including 30 mM UO₂(NO₃)₂ and 105 mM Na₂CO₃ at pH (1) 3, (2) 11, (3) 6 (24 h), and (4) 6 (12 days). (A) Collected normal Raman spectra, (B) barcode, and (C) spectral peak fitting analyses and experiment conditions: excitation wavelength λ_ex = 785 nm; t_int = 20 s; P = 80 mW, and 10 averages. Uranium vibrational frequencies: 871 $({UO}_{2}^{2 +})$ , 859 (UO₂)₂OH³⁺), 853 $({({UO}_{2})}_{2} {(OH)}_{2}^{2 +})$ , 836 $({UO}_{2} {({CO}_{3})}_{2}^{2 -})$ , and 814 $({UO}_{2} {({CO}_{3})}_{3}^{4 -})$ .cm⁻¹. Reprinted with permission from Ref. [49]. Copyright 2016 American Chemical Society.

Additional studies are necessary to confirm previously reported speciation modeling and can be enhanced by pairing vibrational spectroscopy with other techniques. Lucks et al. employed chemometric analysis to reaffirm speciation for the uranyl acetate and succinate systems. An iterative transformation factor analysis on ATR-FTIR spectra confirmed the assignments originally delineated by Ahrland [251], but a new 1:3 U:succinate complex was detected that alters the speciation diagram within the system. This study also highlights a problem in using vibrational spectroscopy to analyze uranyl species. That is, there can be discrepancies between speciation determined from experimental spectra analysis and thermodynamic predictions [48,155,252]. To overcome these limitations, vibrational spectroscopy will need to be paired with experimental measurement techniques, such as potentiometry, X-ray spectroscopy, time-resolved fluorescence spectroscopy, and computational analysis. This will be particularly important as the complexity of the system increases to provide real-world analysis of environmental and biological matrices.

6.2. In situ detection and monitoring of intermediate and metastable uranium compounds

In situ detection of uranium can be successfully achieved using Raman microscopy. As such, changes in uranyl spectral features can be used to identify intermediate and metastable uranyl species. In addition, the combination of Raman microscopy with other instrumental techniques including XPS, X-ray crystallography, and EXAFS can be used to identify metastable uranium compounds. For example, uranium peroxide species undergo thermal decomposition that can be monitored using Raman (λ_ex = 488 nm), IR, and NMR spectroscopies [254]. Spectral changes indicated that uranium is transformed from UO(UO₂(O₂)(H₂O)₂·2H₂O into UO₃ as temperature increases from 30 to 500 °C (Fig. 14). The vibrational frequency of UO₄ H₂O, for instance, is centered at 818 cm⁻¹ when the temperature is between 25 and 50 °C. As the temperature increases beyond 120 °C, a new Raman band appears at 748 cm⁻¹. This vibrational band continues to broaden as temperature increases and red-shifts from 748 to 707 cm⁻¹ as UO_3x(OH)_2x·zH₂O forms. The vibrational bandwidth of the feature at 707 cm⁻¹ is wider than the original band at 818 cm⁻¹ as the crystalline structure is converted into an amorphous form [254]. As such, vibrational spectroscopy is a powerful tool for tracking the properties of uranyl species.

Fig. 14. — Thermal decomposition of uranyl peroxide, UO₄·4H₂O, is monitored by Raman spectroscopy from 30 to 500 °C. The vibrational frequencies of symmetric stretching of U—O are (1) 863, (2) 828, and (3) 818 cm⁻¹. Vibrational frequency of peroxide anion is centered at 748 cm⁻¹ at 120 °C and (4) 675 cm⁻¹ at 400 °C. Figure is reproduced with permission from Ref. [254]. Copyright 2017 Elsevier.

The dynamic uranium speciation after irradiation by alpha-radiation can also be monitored using Raman spectroscopy. For instance, the transformation of uranium (IV) dioxide (UO_2(s)) into uranium peroxide occurs in the presence of water and oxygen [76]. The growth of studtite UO(UO₂(O₂)(H₂O)₂·2H₂O and metaschoepite (UO₂·2H₂O) are observed at 820/870 and 840 cm⁻¹, respectively. These species form upon hydrogen peroxide generation from water irradiation in the presence of oxygen. UO(UO₂(O₂)(H₂O)₂·2H₂O, the product of irradiated UO₂, is similar to the products that form after UO₂ is dissolved in hydrogen peroxide, water, and oxygen. The time-dependent in situ formation of UO₂O₂·4H₂O and UO₃·2H₂O was monitored using Raman spectroscopy (λ_ex = 632.8 nm) without affecting reaction process [76]. Furthermore, oxidation of UO₂ solid to polycrystalline U₃O₈ occurs at 300 K in the presence of water (Fig. 15) [67]. This reaction was monitored successfully and confirmed using Raman spectroscopy (λ_ex = 488 and 785 nm). The valency of uranium was confirmed using XPS. Raman bands for U₃O₈ were observed at 756 cm⁻¹ (combination band of 340 (A_1g) and 405 cm⁻¹ (A_1g)). Speciation assignments were validated from the oxidation state of surface and bulk UO₂ [67].

Fig. 15. — *In situ* Raman spectra of the oxidation of UO₂ (458 cm⁻¹) to U₃O₈ (756 cm⁻¹) by injecting H₂O vapor ranging from 0, 1, 2, and 3 mL. Inset shows the increase of 756 cm⁻¹ (U₃O₈) band increases with the volume of H₂O. Experiment condition: excitation wavelength λ_ex = 488 nm; integration time: 20 s. Figure is reproduced with permission from Ref. [67]. Copyright 2005 Elsevier.

6.3. Extending vibrational spectroscopy to evaluate newly discovered uranium species

Vibrational spectroscopy assists in the in situ identification of solid uranium samples because the observed features in vibrational spectra including vibrational frequencies, FWHM, and relative signal intensities provide information related to the coordination environment, crystallinity, and relative abundance of each uranium species. For example, vibrational frequencies for uranium shift from 445 to 870 cm⁻¹ when the valence number of uranium increases from 4+ to 6+ [66,152]. As a result, vibrational spectroscopy is capable of verifying the identity of known synthesized products [255–257] and newly discovered uranium species such as U (II) [258,259]. Additional Raman spectroscopy measurements were reported for characterizing one-dimensional uranium polymers [255], complex uranyl copper hybrid compounds [257], and uranyl sulfide species [(UO₂)S₄]⁶⁻ [256], These results support that coordination and structural changes from mono- to bi-dentate or transformations variations in coordination bonds between uranyl and ligands such as from U—O to U—S bond formation in the equatorial plane of uranyl can be monitored successfully using vibrational spectroscopy.

6.4. Use of advanced Raman spectroscopy techniques (resonance Raman and SERS)

Inherently, Raman scattering is an inefficient process, which limits the detectability of molecules. As a result, both resonance Raman spectroscopy, a method where excitation energies are similar in energy to electronic transitions in a molecule are utilized, and surface-enhanced Raman scattering (SERS), a method whereby Raman signals can be increased by 6–8 orders of magnitude via interactions with metallic nanostructures, are useful for detecting uranium species. Resonance Raman spectroscopy was applied to develop Raman excitation profiles (REPs) for uranyl nitrate [260,261], acetate [262], chloride [263] and formate [260] in dimethyl sulfoxide (DMSO). As shown in Fig. 16, DMSO is utilized as both the solvent and internal standard. The obtained vibrational frequencies of these uranyl species include ground and excited state resonance Raman features. Vibrational frequencies of excited state species are much more sensitive to changes in the number of ligands and surrounding environment compared to species in their ground state. For instance, the probability of observing uranyl nitrate in an excited state increases as the number of ligands increases from 2 (UO₂(NO₃)₂) to 3 (RbUO₂(NO₃)₃). This causes the vibrational frequency to blue-shift from 703 to 740 cm⁻¹ (Δν₁ = 37 cm⁻¹) [260,261,263]. As a comparison, the vibrational frequencies of ground state species red-shift by 1 cm⁻¹, that is, from 835 to 834 cm⁻¹ (Δν₁ = −1 cm⁻¹) when the number of coordinating ligands increase from 2 to 3. Vibrational frequencies obtained from resonance Raman spectroscopy are sensitive to changes in coordination number; therefore, this technique is an alternative approach for characterizing uranyl species.

Fig. 16. — (A) Resonance Raman spectrum of Cs₂UO₂Cl₄ in DMSO. Experiment condition: excitation wavelength λ_ex = 528.7 nm. Vibrational frequencies: 830 cm⁻¹ $({UO}_{2} {Cl}_{4}^{2 -})$ and 1044.1 cm⁻¹ (DMSO). (B) Magnified spectra of Fig. 5A from 1200 to 700 cm⁻¹. Experiment condition: excitation wavelength λ_ex = (1) 484.5 and (2) 528.7 nm. Figure is reproduced with permission from Ref. [264]. Copyright 2001 Elsevier.

While vibrational spectroscopy is a powerful method for detecting uranium, detection limits do not provide information on trace levels of uranium. To improve the detectability of uranium using vibrational spectroscopy, plasmonic nanomaterials can be used to enhance vibrational band signals thus lowering detection limits. To date, surface enhanced Raman scattering (SERS) is the most widely used technique for this purpose. For instance, the detection of uranium was exploited with different SERS substrates including both silver and gold nanostructures. Silver nanomaterials, in general, provide larger signal enhancements versus gold due to the inherent metal permittivity [265]. In addition, native oxide layers that form on silver promote the adsorption of uranyl via oxophilic interactions [237,266,267]. This was confirmed from SERS spectra where a UAO symmetric stretch was observed at 730 and 710–700 cm⁻¹ on silver [266] and silver doped sol-gel substrates [267], respectively.

Uranyl was successfully detected down to 85 nM when silver doped sol-gel materials were used. Gold nanomaterials exhibit superior chemical stability versus silver and have also been employed for uranyl detection [237,268,269]. Because gold lacks a native oxide layer, gold nanomaterials must be functionalized with molecules that coordinate to uranyl for SERS detection. In general, this strategy improves uranyl detectability on both silver and gold nanomaterials [268–271]. For instance, N-(2-mercaptopropionyl)glycine (MPG) was assembled onto silver SERS substrates because both amide and carboxyl groups can coordinate with uranyl. By doing this, a SERS signal was observed at ~840 cm⁻¹, and a detection limit of 5 nM was achieved [270].

Gold nanomaterials can be modified with many different terminal functional groups. For instance, carboxylate terminated alkanethiols were selected to modify gold nanostars for uranyl detection. The carboxylate groups coordinated to uranyl and caused the vibrational frequency of uranyl to be centered at ~840–830 cm⁻¹. Quantitative SERS detection of uranyl was also achieved using gold nanomaterials [269]. Here, (aminomethyl) phosphonic acids facilitated interactions between uranyl and gold [268] through phosphate-uranyl coordination. This resulted in a vibrational mode centered at 830 cm⁻¹ and selective uranyl detection from a 10 mM NaHCO₃ and humic acid solution.

To improve the robustness of SERS detection for uranyl, nano-materials can be dispersed onto supports. For instance, silver nanoparticles were deposited onto reduced graphene oxide nanosheets to form SERS substrates [272]. This led to efficient uranyl adsorption via electrostatic interactions between the cationic uranyl species and anionic oxide groups on graphene oxide. SERS signals for uranyl acetates were shown to red-shift from 838 (Fig. 15A) to 714 cm⁻¹ (Fig. 17B) from a pH 5 solution upon adsorption to the substrate, respectively. These vibrational frequency changes indicate that charge transfer from graphene oxide to uranyl occurs once uranyl adsorbs to silver. This approach showed that uranyl detection was reproducible, and detection limits down to 10 nM were demonstrated. Accordingly, applying SERS for the identification and quantification of uranyl is a promising avenue for uranyl detection opportunities.

Fig. 17. — (A) Normal Raman spectrum of 5 mM uranyl acetate and (B) SERS spectra of 10 μM of uranyl acetate on silver doped reduced graphene oxide nanosheets. Figure is reprinted with permission from Ref. [272] Copyright 2013 American Chemical Society.

6.5. Conclusions and outlook

In summary, both infrared and Raman spectroscopies are powerful techniques for the detection and identification of uranium species. Importantly, unique vibrational frequencies generally correlate to specific uranyl coordination environments providing a robust method for identification of solid compounds, solution complexes, and surface species. Additionally, vibrational frequencies for uranyl species in both solid and solution phases can be used to estimate force constants, bond lengths, and/or coordinating ligand composition. Vibrational frequencies for uranyl species adsorbed on mineral surfaces and sorbents were summarized as were those for aqueous uranyl species. Vibrational spectroscopy is highly suitable for the in situ detection of uranium species; however, overlapping bands can result in poorly assigned uranyl species found in samples. This can limit uranyl detectability because Raman spectral cross sections are small. In cases where significant signals are detected, spectral deconvolution techniques can be employed as can nanomaterials so that Raman signals are further enhanced.

Continued development of instrumentation and data analysis procedures will lead to improvements in the identification, detectability and understanding of uranium species using vibrational spectroscopy. These tools will provide more robust methods for the detection of uranium in the environment, provide new opportunities for nuclear forensics applications, and provide a better understanding of separations processes. The development of these techniques for uranium can also then be translated to other actinides (Np, Pu, Am) that are relevant within the nuclear fuel cycle. A detailed evaluation of how the spectral bands change along the 5f-series can also provide additional information on electronic structure and bonding within this complex family of elements.

8.

Funding sources

T.Z.F. acknowledges support from the U.S. Department of Energy, Basic Energy Sciences, Heavy Elements Chemistry Division grant (DE-SC0013980) regarding the sections associated with the solid-state uranium compounds and minerals. A. J. H. was supported by the National Institute of Environmental Health Sciences of the U.S. National Institutes of Health under award number R01ES027145.

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PERMALINK

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

Grace Lu

Amanda J Haes

Tori Z Forbes

Abstract

1. Introduction to uranium chemistry

Table 1.

2. Detection and identification of uranium using vibrational spectroscopy

2.1. Vibrational spectroscopy of U(IV)

2.2. Vibrational spectroscopy of U(VI)

Fig. 1.

Fig. 2.

2.3. Vibrational spectroscopy of U(V)

3. Chemical and structural elucidation of uranium solid-state compounds

3.1. Mineral phases and inorganic uranium compounds with extended topologies

Table 2.

3.1.1. Uranium oxide, peroxides, hydroxides, and halides

3.1.2. Uranium compounds and minerals containing oxyanions

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

3.2. U(VI) coordination compounds and hybrid materials

Table 3.

Fig. 7.

3.3. Spectroscopy as predictive tools for solid state bonds

Fig. 8.

4. Chemical identification and dynamic studies of soluble uranyl species (UO22+)

4.1. Uranyl species in aqueous solutions

Table 4.

Fig. 9.

Fig. 10.

Fig. 11.

4.2. Uranyl species in ionic liquids

4.3. Uranyl species in polar organic solvents

4.4. Using vibrational spectroscopy to provide thermodynamics of solution-phase species

5. Surface speciation studied by IR and Raman spectroscopies

5.1. Uranyl adsorption onto oxide surfaces

Fig. 12.

5.2. Identification of insoluble aqueous species on solid-state surfaces

5.3. Monitoring uranium redox using vibrational spectroscopy

5.4. Changes in Raman bandwidths on solid state surfaces

5.5. Spectral signals collected on functionalized solid materials

6. Challenges, opportunities, and vision

6.1. Uranium detection in complex aqueous matrices

Fig. 13.

6.2. In situ detection and monitoring of intermediate and metastable uranium compounds

Fig. 14.

Fig. 15.

6.3. Extending vibrational spectroscopy to evaluate newly discovered uranium species

6.4. Use of advanced Raman spectroscopy techniques (resonance Raman and SERS)

Fig. 16.

Fig. 17.

6.5. Conclusions and outlook

8.

References

ACTIONS

PERMALINK

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4. Chemical identification and dynamic studies of soluble uranyl species $({UO}_{2}^{2 +})$