Synthesis, Characterization, and Stability of Two Americium Vanadates, AmVO₃ and AmVO₄

Abstract

In search for chemically stable americium compounds with high power densities for radioisotope sources for space applications, AmVO₃ and AmVO₄ were prepared by a solid-state reaction. We present here their crystal structure at room temperature solved by powder X-ray diffraction combined with Rietveld refinement. Their thermal and self-irradiation stabilities have been studied. The oxidation states of americium were confirmed by the Am M₅ edge high-resolution X-ray absorption near-edge structure (HR-XANES) technique. Such ceramics are investigated as potential power sources for space applications like radioisotope thermoelectric generators, and they have to endure extreme conditions including vacuum, high or low temperatures, and internal irradiation. Thus, their stability under self-irradiation and heat treatment in inert and oxidizing atmospheres was tested and discussed relative to other compounds with a high content of americium.

Short abstract

AmVO₃ and AmVO₄ were prepared by a solid-state reaction and characterized in terms of crystal structure, americium and vanadium oxidation states, thermal behavior, and self-irradiation stability. With a fast amorphization behavior, large volume variation, and interchangeable structure as a function of the presence of the oxygen atmosphere and temperature, americium vanadates do not appear to be good candidates as Am forms for radioisotope power systems.

1. Introduction

The americium isotope ²⁴¹Am is formed during storage of plutonium via β^–-decay of ²⁴¹Pu with a half-life of 14.33 years. Due to ²⁴¹Am accumulation in existing stocks of civil-separated plutonium in Europe, and its relatively high specific power of 0.114 W/g, ²⁴¹Am has been proposed for use in radioisotope power systems (RPSs)¹ and is under consideration by the European Space Agency (ESA) as an energy source for future European space missions.² The requirements for a stable solid form are very diverse, ranging from storage on earth and operation in space to safe performance in case of accidents and post-accident scenarios. Our group studied several ceramic forms containing significant specific Am amounts,³⁻⁵ the present study being in line with the past efforts.

The synthesis and crystal structure of AmVO₃ were reported for the first time by Keller and co-workers.^6,7 In their work, AmVO₄ was prepared by dissolving AmO₂ and V₂O₃ as hydroxides, mixed, dried, and reacted at 1250 °C under an oxidizing atmosphere. The produced AmVO₄ was subsequently reduced to obtain AmVO₃. The authors proposed the crystal structure for the two compounds (perovskite-type for AmVO₃ and zircon-like for AmVO₄) and gave the lattice parameters.

According to the Inorganic Crystal Structure Database (ICSD), there are no compounds of americium and vanadium with a fully characterized crystal structure. We present here the complete crystal structure refinement of the two vanadates by Rietveld analysis at room temperature. Spectroscopic and microscopic studies were carried out to check the purity of the samples. The high-resolution X-ray absorption near-edge structure (HR-XANES) technique was applied at the Am M₅ edge to characterize the Am oxidation states. The stability of AmVO₃ and AmVO₄ under self-irradiation and heat treatment in an inert and oxidizing atmosphere was tested and discussed relative to other americium-containing compounds.

2. Experimental Section

Caution: ²⁴¹Am is a highly radioactive isotope (t_1/2 = 432.8 years, specific activity of 126.8 GBq/g). All work presented in this paper were carried out in glove boxes in radiological laboratories licensed for handling actinides. When appropriate, shielding and remote handling tools were used to protect the workers in these experiments.

2.1. Sample Preparation

2.1.1. AmVO₃

Stoichiometric amounts of ²⁴¹AmO₂ (158 mg of aged material containing about 7% ²³⁷Np as the decay product and 2% ²³⁹Pu) and V₂O₅ (48 mg, Merck, 99.95%) were mixed in an agate mortar and pressed into a pellet. The thermal treatment was performed for 10 h at 1250 °C under Ar/H₂ (5%) with heating and cooling ramps of 200 °C/h.

2.1.2. AmVO₄

A chip of 78 mg of the previous AmVO₃ pellet was oxidized for 2 h up to 1000 °C under oxygen by using a Netzsch STA 449C thermogravimetric analysis instrument. Oxidation started at about 400 °C with a maximum oxidation rate at 450 °C, as discussed in the Materials Characterization section. This synthesis was done independently from the thermogravimetric analysis described below during in which higher temperatures were reached.

2.2. Thermogravimetric Analysis

The thermal behaviors of AmVO₃ and AmVO₄ were assessed under an argon/hydrogen and oxygen atmosphere, respectively (to preserve the oxidation state of the respective compounds) using a Netzsch STA 449C thermogravimetric analysis instrument. The temperature was controlled by a Pt-PtRh (10%) thermocouple. The measurements were conducted on pellet fragments (11–78 mg) up to 1500 °C in alumina crucibles, and the applied heating and cooling rates were 10 °C/min.

2.3. XRD

Room temperature XRD analyses were performed on about 10 mg of powdered material loaded in a bicomponent epoxy resin using a Bruker D8 Advance diffractometer (Cu Kα radiation, 40 kV, 40 mA) mounted in a Bragg–Brentano configuration. The diffractometer was equipped with a curved Ge(1,1,1) Kα₁ monochromator, a ceramic copper tube, and a LinxEye position sensitive detector. The XRD patterns were recorded using a step size of 0.01° across the 10° ≤ 2θ ≤ 120° angular range. Structural analysis was performed by the Rietveld method using the Jana2006 software.⁸

Since all the crystallographic parameters could not be independently refined, some structural constrains were applied. For the atomic displacement parameter, isotropic displacement parameters U_iso were used, and the following equation has been considered:⁹

1

The V–O distances were constrained to 2 Å for the perovskite structure AmVO₃¹⁰ and to 1.71 Å for the zircon structure AmVO₄.¹¹

2.4. Raman Spectroscopy

Raman spectroscopy measurements were performed on fragment of pellets (2–3 mg) at room temperature and ambient pressure on a polycrystalline specimen using a Horiba Jobin-Yvon T64000 spectrometer. For technical reasons, two lasers at different wavelengths were used; a 647 nm Kr+ laser and a 660 nm solid state laser excitation source. A 50× long focal objective was used to irradiate the sample and collect the back-scattered light. Great care was taken to avoid sample damage or laser-induced heating. Measurements were performed with few tenths of a milliwatt incident power.

2.5. Scanning Electron Microscopy

Images of the samples (fragment of pellets of 1–2 mg) were obtained in a Philips/FEI XL40 SEM operated at 25 kV, equipped with a SAMx energy-dispersive X-ray analysis system (EDX). This microscope (high-voltage unit, column, chamber, and turbomolecular pump) was placed inside a glovebox, while the components that are not getting in contact with the active materials (primary vacuum system, the water-cooling circuit, and the acquisition electronic) were outside.¹²

2.6. XANES and HR-XANES

For AmVO₃ and AmVO₄ compounds, the Am M₅ HR-XANES spectroscopy technique was performed at the ACT station of the beamline for catalysis and actinide research (hereafter CAT-ACT beamline) of the KIT Light Source, Karlsruhe, Germany.¹³ Spectra acquisitions were done utilizing a Johann type X-ray emission spectrometer. The incident beam was monochromatized by a Si(111) double-crystal monochromator (DCM), focused to 500 × 500 μm, and subsequently narrowed down by slits onto the sample to a spot size of about 200 × 200 μm. The X-ray emission spectrometer consists of four Si(220) crystals with a 1 m bending radius and a single diode VITUS silicon drift detector (Ketek, Germany), which together with the sample were arranged in a Rowland circle geometry.¹³ AmO₂ reference sample was used to calibrate Am M₅ HR-XANES spectra. The main absorption maximum was set to 3890 eV for AmO₂.¹⁴ The maxima of the WLs are located at AmCl₆^3– (3888.4 eV), AmFe₂ (3887.5 eV) reported elsewhere.^15,16 Both Am M₅ edge data cited above were measured in conventional fluorescence mode and were similar to AmVO₃ and AmVO₄ (3888.5 eV) recorded in this work. The slight divergence of the absorption maximum of AmO₂ in the current work (3890 eV) from the data reported by Epifano et al.(17) (about 3891.5 eV) is a result of different experimental energy resolutions (for details cf. Discussion in the Supporting Information and Figure S5). The multi-position sample cell (containing <1 mg powdered material embedded in bicomponent glue) was placed into a double containment, where the inner compartment was sealed by 8 μm and the outer compartment by 13 μm Kapton foil. The HR-XANES spectra were measured with a step size of 0.1 eV from −10 to +25 eV from the white line (WL) of the respective edge and 0.5 eV in all other parts of the spectra. At least two spectra were averaged for each sample. The sample, crystals, and detector were enclosed in a box filled with helium to minimize intensity losses due to scattering and absorption of photons in air. Constant helium flow was maintained to keep the oxygen level below 0.1%.

Additionally, Am L₃ edge and V K-edge XANES measurements were performed at the INE-Beamline¹⁸ of the KIT Light Source, Karlsruhe, Germany. The radiation protection measures were kept identical to those used at the ACT station.¹⁹ Two Ge(422) and two Si(111) crystals were mounted in the double-crystal monochromator (DCM) for Am L₃ and V K edge measurements, respectively. The beam was focused on a ∼0.5 mm × 0.5 mm spot on the sample. Zr or V metal foils were used for energy calibration for the Am L₃ edge or V K edge XANES, respectively. V₂O₃ (Sigma-Aldrich, 99.99%) and V₂O₅ (Merck, 99.95%) powders were mixed with cellulose and pressed into the pellets and used as references for V K edge XANES. The XANES spectra were measured in fluorescence mode with step sizes of 0.25 and 0.8 eV from −10 to +25 eV from the white line (WL) of the respective edges for the V K edge and Am L₃ edge and 4 eV steps in the post-edge area of the spectra. At least two spectra were averaged for each sample. Measurements were performed in air, and no radiation damage was observed during the measurements.

2.7. α Self-Irradiation

Due to the high alpha activity of americium, the doses accumulated by the AmVO₃ and AmVO₄ at the time of different measurements must be considered. Daily doses of about 7.8 × 10¹⁵ and 7.4 × 10¹⁵ α/g were built up for AmVO₃ and AmVO₄, respectively. Only TGA and XRD techniques could be applied on freshly prepared material. The calculated doses of the materials at the time of the different measurements are summarized in Table 1. Between measurements, the samples were stored under the atmosphere of the glovebox where the samples were produced (nitrogen with up to 10,000 ppm oxygen).

Table 1. α Dose Accumulated in AmVO₃ and AmVO₄ at the Time of the Different Measurements, in α Decay Events per Gram of Material.

	AmVO3		AmVO4
	sample age, days	cumulated dose, α/g	sample age, days	cumulated dose, α/g
XRD	1.6	1.2 × 1016	2.7	2.1 × 1016
	132	1.0 × 1018	131	1.0 × 1018
	212	1.6 × 1018	211	1.6 × 1018
TGA	3.4	2.7 × 1016	2.5	1.9 × 1016
Raman	451	3.5 × 1018	450	3.5 × 1018
	890	6.8 × 1018	890	6.8 × 1018
SEM	660	5.1 × 1018	659	5.1 × 1018
Am L3 edge XANES, Am M5 edge HR-XANES	603	4.7 × 1018	607	4.7 × 1018
V XANES	726	5.6 × 1018	727	5.6 × 1018

3. Results and Discussion

3.1. Materials Characterization: Fresh Material

During the synthesis of AmVO₄ by oxidation of an AmVO₃ specimen, the DTA measurement (Figure S1) performed in oxygen indicated a single exothermic event reaching a maximum energy release at 450 °C with an associated weight gain of 4.5 wt %, which was attributed to the oxidation to the AmVO₄ form (expected gain of 4.7 wt %).

Americium vanadate AmVO₃ crystalizes in the perovskite Pbnm structure after sintering under an Ar/H₂ atmosphere; meanwhile, after thermal treatment in oxidative conditions, the material converts into AmVO₄ showing a zircon-like I4₁/amd structure (Figure 1 and Table 2). Residual NpO₂ can be detected in quantifiable amounts, which indicates that this daughter element of ²⁴¹Am does not fully integrate the two vanadate compounds and segregates (at least partially) into an oxide phase. The Np L₃ edge XANES data are shown in Figure S4, supporting the above statement.

Figure 1.

XRD patterns, Rietveld refinement, and crystal structure of AmVO₃ and AmVO₄.

Table 2. Unit Cell Parameters and Results of the Rietveld Refinement for AmVO₃ and AmVO₄^a.

formula unit	AmVO3	AmVO4	AmVO4 (Keller et al.(7))	AmVO4 (Goubard et al.(20))
space group	Pbnm	I41/amd	I41/amd	I41/amd
space group number	62	141	141	141
a, Å	5.4445(3)	7.2938(2)	7.31(1)	7.311(2)
b, Å	5.5914(3)	7.2938(2)	7.31(1)	7.311(2)
c, Å	7.7556(4)	6.4284(2)	6.42(1)	6.422(2)
α, °	90	90	90	90
β, °	90	90	90	90
γ, °	90	90	90	90
V, Å3	236.10(2)	341.992(10)	343	343.26
Z	4	4	4	4
M, g/mol	340.00	356.00	356.00	356.00
crystallographic density, g/cm3	9.565	6.9142	6.89	6.89
GOF	1.57	2.15
RP	0.0106	0.0154
Rwp	0.0150	0.0254
Rexp	0.0068	0.0072

^aR_p = Σ(i)[|yi(obs) – yi(calc)|]/Σ(i)[yi(obs)]; R_wp = {Σ(i)[wi(yi(obs) – yi(calc))²]/Σ(i)[wi × yi(obs)²]}^1/2; GOF = R_p/R_exp; R_exp = {Σ(i)[wi × yi(obs)²]/(n – p)}^1/2.

The crystal structures suggest that americium stays in the oxidation state III in the materials whatever the redox condition, while vanadium can have the oxidation state III or V, which results in ABO₃ perovskite or ABO₄ zircon structures, respectively. One can notice that during the oxidation process, the density of the material strongly decreases from 9.565 g/cm³ for the close-packed AmVO₃ to 6.914 g/cm³ for the oxyanionic AmVO₄, which indicates a volume increase of 45% during oxidation. The formation of a zircon-like structure for AmVO₄ is in accordance with the works of Keller et al.(6,7) or of Goubard et al.(20) showing slightly lower but similar lattice parameters (Table 2).

TGA analyses, under air and argon/hydrogen for AmVO₄ and AmVO₃, respectively, revealed no significant weight loss for both materials, indicating a good thermal stability of the compounds at high temperature. However, XRD analysis of AmVO₃ after TGA measurement showed the presence of a new unidentified phase, AmVO₃ remaining as the main compound, suggesting the beginning of a degradation of the material at this temperature. In contrast, XRD measurement of AmVO₄ after TGA showed that the material remains fully unchanged, indicating an excellent thermal stability of this material under oxidative conditions.

The evolution of the unit-cell parameters/volumes of the MVO₃ and MVO₄ compounds (Figure 2) confirms that the Am-vanadates belong to the corresponding crystallographic families. Only the perovskite phases crystalizing in the orthorhombic Pbnm space group are considered in the MVO₃ part of Figure 2. The atomic coordinates and displacement parameters for AmVO₃ and AmVO₄ compounds are presented in Table 3.

Figure 2.

Evolution of the unit-cell volumes of MVO₃ and MVO₄ compounds as a function of the cationic radii. The unit-cell volume values are taken from the Inorganic Crystal Structure Database (https://icsd.fiz-karlsruhe.de) and the ionic radii from Shannon.²¹

Table 3. Atomic Coordinates and Displacement Parameters for AmVO₃ and AmVO₄ Compounds.

element	label	oxidation state	SOF	Wyckoff position	x	y	z	Uiso, Å2
AmVO3 (Pbnm, perovskite)
Am	Am1	+3	1	4c	0.9888(15)	0.0507(5)	1/4	0.004(1)
V	V1	+3	1	4b	1/2	0	0	0.004(1)
O	O1	–2	1	4c	0.397(8)	–0.010(9)	1/4	0.008(2)
O	O2	–2	1	8d	0.745(12)	0.263(11)	0.037(6)	0.008(2)
AmVO4 (I41/amd, zircon)
Am	Am1	+3	1	4a	0	3/4	1/8	0.0074(6)
V	V1	+5	1	4b	0	1/4	3/8	0.0074(6)
O	O1	–2	1	16h	0	0.4377(11)	0.2156(17)	0.0148(12)

3.2. Materials Characterization: α Self-Irradiated Material

Due to the impact of the COVID-19 pandemic, SEM, XANES, and Raman characterization could only be performed about 1–2 years after synthesis. Due to the high alpha activity of ²⁴¹Am, the effect of self-irradiation must be considered in this condition, the main effect being amorphization of the crystal structure. The amorphization of the two vanadates was followed through XRD analyses, confirming that SEM, XANES, and Raman were performed on amorphous material. However, the results further described below seem to show only a limited impact of this amorphization on the microstructure (SEM) and electronic structure (XANES) of the two vanadates, while Raman spectroscopy showed limited impact of self-irradiation for AmVO₄ and a chemical transformation due to Raman laser heating for AmVO₃.

The XRD results reveal that the volume of the close-packed AmVO₃ perovskite structure increases by about 4.2% after a dose of 1.03 × 10¹⁸ α/g, while that of the oxyanionic AmVO₄ zircon structure shows a contraction of about 1.8% after a similar dose (Table 4). Therefore, the increased lattice disorder under α self-irradiation in this low-density structure results in a more compact arrangement. One can clearly see the swelling in AmVO₃ and the contraction in AmVO₄ in Figure 3, with a shift of the diffraction peaks into lower and higher 2Θ values, respectively. After a dose of about 1.65 × 10¹⁸ α/g (7 months), the main diffraction peak of AmVO₃ is still visible, which indicates that the amorphization process is well advanced but not completed, while at that dose, the amorphization of AmVO₄ seems fully completed.

Table 4. Lattice Parameter Variation of AmVO₃ and AmVO₄ under Self-Irradiation.

a, Å	b, Å	c, Å	V, Å3	volume variation, %	dose (α/g)	dpaa
AmVO3 (Pbnm, perovskite)
5.444	5.590	7.755	235.982		0.01 × 1018	0.0016
5.513	5.661	7.882	245.994	4.2	1.03 × 1018	0.158
AmVO4 (I41/amd, zircon)
7.294	7.294	6.428	341.952		0.02 × 1018	0.0027
7.243	7.243	6.405	335.964	–1.8	1.02 × 1018	0.134

^aEstimated from the SRIM2013 simulation of displacements produced by an ²⁴¹Am alpha-decay (i.e., an alpha-particle of 5.486 MeV and a ²³⁷Np recoil nucleus of 92 keV) in AmVO₃ and AmVO₄ yielding dose multiplication factors, d, from dose to displacement per atom of 1700 and 1800, respectively.

Figure 3.

Evolution of the XRD pattern of AmVO₃ and AmVO₄ over time showing the progressive amorphization of the material.

The SE micrographs presented in Figure 4 were recorded about 2 years after the synthesis. Even if the specimens had a relatively high level of self-irradiation at the time of the measurements, it can be observed that the microstructures are typical for solid state reactions, confirming the self-homogenization of the reaction mixture during heat treatment.

Figure 4.

SE images of the microstructure of AmVO₃ (left) and AmVO₄ (right) recorded at 660 days after synthesis (5.1 × 10¹⁸ α decay events/g).

To assess the oxidation state of both americium and vanadium in the AmVO₃ and AmVO₄, XANES spectroscopy characterization was performed. Am M₅ edge HR-XANES, Am L₃ edge XANES, and V K edge XANES spectra were collected. The HR-XANES data at the Am M₅ edge reveals that Am atoms have an Am^III oxidation state for both AmVO₃ and AmVO₄ compounds. The energy position of the main absorption resonance (white line, WL) maximum in the AmVO₃ and AmVO₄ spectra (Figure 5, blue and red solid lines) is located at about 3888.5 eV that is characteristic of the main absorption intensity of Am^III, consistent with previously reported data.^14,15 Note that a very limited number of Am M₅ data are published to date. To be able to compare to published spectroscopic data on compounds containing Am^III, Am L₃ edge XANES experiments were also performed prior to collecting the V K-edge data. The AmVO₃, AmVO₄, and AmO₂ XANES spectra were obtained following background subtraction by fitting a linear polynomial to the pre-edge region of the absorption spectrum and normalized at the maximum (WL) intensity for Am M₅ (Figure 5) and at the high energy range for Am L₃ and V K edge data (Figures 6 and 7). The maxima of the WLs located at Am^IIIVO₃ (18,521 eV eV), Am^IIIVO₄ (18,521 eV), and Am^IVO₂ (18,525 eV) are similar to Am^III₂O₃ and Am^IVO₂ reported elsewhere.²² The difference in the tail feature around 18,533 eV as well as smearing of the shape-resonance located at about 18,556 eV for AmVO₃ as compared to AmVO₄ suggests a different level of amorphization. Therefore, even though at this level of self-irradiation, both AmVO₃ and AmVO₄ are lacking a long-range order as observed by XRD, the degree of amorphization is somewhat different as revealed by Raman data (see below) and suggests that amorphization affects less the local structure in AmVO₄ as compared to AmVO₃.

Figure 5.

HR-XANES spectra acquired at the Am M₅ edge for AmO₂ (green, dash-dotted), AmVO₃ (red, solid, 4.7 × 10¹⁸ α/g), and AmVO₄ (blue, solid, 4.7 × 10¹⁸ α/g) materials. The maximum of the white line position is indicated for Am^III and Am ^IV compounds, recorded at 3888.5 and 3890 eV, respectively.

Figure 6.

XANES spectra acquired at the Am L₃ edge for AmO₂ (green, dash-dotted), AmVO₃ (red, solid, 4.7 × 10¹⁸ α/g), and AmVO₄ (blue, solid, 4.7 × 10¹⁸ α/g) materials. The shift of the WL maximum of 4 eV is consistent with the chemical shift between Am^III and Am^IV.

Figure 7.

XANES spectra acquired at the V K edge for AmVO₃ (red, solid, 5.6 × 10¹⁸ α/g) and AmVO₄ (blue, solid, 5.6 × 10¹⁸ α/g) along with V^III (orange, dotted) and V^V (black, dashed) references.

To directly compare the electronic structure of vanadium, the background-subtracted and normalized V K-edge XANES spectra for AmVO₃ and AmVO₄ (blue and red solid lines, respectively) as well as reference V₂O₃ and V₂O₅ compounds (orange dotted and black dashed lines, respectively) are shown in Figure 7. As expected, for AmVO₃, vanadium has a V^III oxidation state since all spectral features (A–D) match the spectral features of the V₂O₃ reference spectrum. The reduced intensity of the pre-edge feature A in AmVO₃ compared to V₂O₃ may indicate changes in the electronic structure of vanadium as a result of AmVO₃ amorphization due to self-irradiation effects as well as the influence of Am-presence in the resulting AmVO₃ after reaction of AmO₂ and V₂O₅ at 1250 °C, which differs from the pure V₂O₅ structure. The V K-edge XANES spectrum of AmVO₄ exhibits a strong pre-edge feature (A) characteristic to V^V that is well known to be due to a formally forbidden (very weak) 1s → 3d electronic transition, which is dipole-allowed when the local O_h symmetry is distorted.²³⁻²⁵ Features B and B′ seem to be smeared out for AmVO₄, indicating a change of the VO₅ square-pyramidal polyhedral configuration in V₂O₅ to tetrahedral in AmVO₄. The findings are consistent with previously reported data by Benzi et al.(26) for V₂O₅ and palenzonaite (natural V^V) compounds. In that study, feature C′ in the tetrahedral geometry is located at about 5510 eV, matching the position of the shape-resonance C′ observed for AmVO₄.

In conclusion, all the oxidation states probed through XANES and HR-XANES measurement are in perfect agreement with the expectations based on the initial crystallographic structure of the two materials. The effect of self-irradiation and amorphization has therefore no impact on their oxidation states.

The Raman spectrum of the aged AmVO₄ (Figure 8) suggests that the VO₄ tetrahedra in the zircon-like structure are intact despite the amorphization of AmVO₄ at this level of self-irradiation found by XRD. The five clearly identifiable internal modes of the [V–O₄] tetrahedron, ν₁(A_1g), ν₃(B_1g,E_g), ν₄ (B_1g), ν₂(A_1g), and ν₂ (B_2g), were found at very similar wavenumbers as for the lanthanide LnVO₄ zircon compounds,²⁷ as shown in Table 5, though somewhat lower than the orthovanadate of neodymium, whose ionic radius is closest to trivalent Am^III. The two clearly identifiable low-frequency external modes, which reflect the motion between the [V–O₄] tetrahedron and the Am^III ion, were observed, of which the lowest is the T(B_1g) and the highest is assigned to the R(E_g). The spectrum also shows a very weak mode at the position of one of the T(E_g) modes. The T(B_1g) mode is substantially lower than that found for the lanthanide LnVO₄ compounds.²⁷ This can be explained by the observation by Moura et al.(28) that the T(B_1g) mode in the Tb(V_1–xP_x)O₄ solid solution shows a different broadening, indicating that it is related primarily to the Tb motion on the [Tb–O₈] sublattice. The strong shift observed here thus supports that it is due to the mass effect between the actinide and lanthanide series.

Figure 8.

Room-temperature Raman spectrum of AmVO₄ recorded at 450 days after synthesis (3.3 × 10¹⁸ α decay events/g).

Table 5. Raman Modes of AmVO₄ (in cm^–1) Compared to Some Lanthanide Analogues^a.

	AmVO4	LaVO4	CeVO4	NdVO4	SmVO4
	this work	Cheng et al.(31)	Panchal et al.(30)	Nguyen et al.(32)	Santos et al.(27)
ν1(A1g)	850	859.0	864.3	871	877
ν3(Eg)	785	796.7	801.3	795	816
ν3(B1g)	785	783.3	789.1	808	816
ν4 (B1g)	457	462.3	468.9	472	477
ν2(A1g)				381	380
ν4 (Eg)	370	376.6	381.1
ν2 (B2g)	255	262.9	261.9	375
T(B1g)					261
R(Eg)	225	228.7	234.1	237	240
T(Eg)	[140]	141.8
T(B1g)	95	119.7	124.4	123	123
T(Eg)		109.5		113	112

^aWe follow the band assignment for CeVO₄ by Panchal et al.(30) that was supported by density functional theory (DFT) calculations.

The shifts of the modes in AmVO₄ compared to the LnVO₄ series are very similar to AmPO₄⁵ and LnPO₄.²⁹ However, the strong broadening of the modes after aging that was observed for AmPO₄ did not occur. This may be explained by recrystallization of the amorphous AmVO₄ phase resulting from heating by the incident Raman laser. This would suggest relatively low critical amorphization temperatures, in line with the observations for the monazite and zircon LnPO₄ analogues.²⁹

Surprisingly, the Raman spectrum of the aged AmVO₃ sample was identical to that of AmVO₄, suggesting that oxidation took place. We exclude that the oxidation is related to the damage build-up since it is not observed in the XRD and XANES results at similar dose. We therefore conclude that it is triggered by heating the laser during the Raman measurement.

3.3. Closer Look at the α Self-Irradiation Mechanism in AmVO₄

Our results for AmVO₄ amorphization derived from XRD analysis differ significantly from the observations in the study of Goubard et al.,²⁰ who studied the radiation damage build-up in AmVO₄ during a much longer period of 5 years (14 × 10¹⁸ α/g). They reported that the structure remained at least partially crystalline during this period and became predominantly amorphous after 5 years, which is about 10 times slower than in our work. Moreover, Goubard et al.(20) did not observe a volume contraction under α self-irradiation like in the present study, but a moderate lattice expansion with a volume increase to a maximum of about ΔV/V₀ = 1.2%. At a comparable dose to our study (1 × 10¹⁸ α/g), they found ΔV/V₀ = 0.15%.

The reasons for the discrepancies with the work of Goubard et al.(20) remain unclear, and one can only hypothesize the following:

• A different content of neptunium (daughter element of americium) in the initial material. Note that Goubard et al.(20) did not give details about the purity of the used Am. The exact quantity of Np that could be incorporated in AmVO₄ is not known, but we observed NpO₂ as the separate phase in this work. Moreover, uptake of Np^IV in the structure would require charge compensation, for example, by V^III, which was not observed by XANES in our work.

• The higher synthesis temperature in the current work (1000 °C vs 600 °C), which could have resulted in a more crystalline material and thus less initial disorder. As a result, a potential time span of contraction could have been absent or short in the study by Goubard et al.(20) It should be noted that their first data point at 0.7 × 10¹⁸ α/g is anomalous in their ΔV/V₀ vs dose curve and indicates a minimal expansion (ΔV/V₀ = 0.03%) after approximately 90 days.

• The difference in production scale (hundreds of milligrams here versus 1 mg by Goubard et al.(20)) and more probable methods (synthesis temperature and duration) could have resulted in substantially different crystallite sizes, powder morphologies, and densities, thus potentially affecting the impact of the alpha decay through annihilation of defects, in particular via diffusion to sinks (grain boundaries).³³

• Radiogenic helium can interact with defects (vacancies), but the effect of this will depend on the microstructure and crystallinity. The larger the grain size, the larger the helium fraction retained in the lattice, affecting the defect recombination kinetics. Similarly, the initial disorder (crystallinity) will affect the helium retention. Unfortunately, information on the microstructure of the material synthesized by Goubard et al.(20) is missing.

The first three hypotheses could explain the slight difference of the initial lattice parameters (see Table 2), but not the different kinetics. Although the damage kinetics could be affected by size and geometry of samples analyzed in the XRD instruments, the discrepancy is huge. The last two hypotheses could explain the different recombination kinetics, but remain speculative, in the absence of comparative microstructural data.

Our observations for zircon-type AmVO₄ are different from our results for the monazite-type AmPO₄.⁵ The zircon structure is closely related to the monazite LnMO₄ structure, both made of isolated tetrahedra of V or P, connected by trivalent metal ions in eight or nine coordinations, respectively. Under the influence of temperature and/or pressure, a transition can take place.^30,31 For AmPO₄, a significant expansion of the unit cell was observed (ΔV/V₀ = 1.8% at 0.6 × 10¹⁸ α/g), whereas the AmVO₄ cell contracts with irradiation time. However, the dose at which the transition to a fully amorphous phase takes place is close in both studies, slightly higher for AmVO₄ compared to AmPO₄, which is in line with the observation of Meldrum et al.(29) that the critical amorphization temperature is slightly higher for zircon-type LnPO₄ compounds as well.

These contrasting observations are not easy to combine. Volume contraction due to α self-irradiation is known for Am₂Zr₂O₇ pyrochlore³⁴ and was attributed to the stability of the [Zr–O₆] octahedra and their rotation in response to the increase in disorder around americium. Although Am₂Zr₂O₇ has a close-packed pyrochlore structure, which is very different from the open oxyanionic structures of zircon and monazite, this explanation may help understand. The volume in the LnVO₄ series is discontinuous, with the zircon (xenotime) structure (eight-coordinated M^III) being larger than the monazite (nine coordination),³⁵ similar to the LnPO₄ series.³⁶ So, in case the radiation damage in zircon-type AmVO₄ affects predominantly the americium coordination sphere leading to a higher coordination with a concomitant rotation of the [V–O₄] tetrahedra, the volume will decrease, as we observed here. In AmPO₄, the Am ions are already nine-fold coordinated, resulting in prompt expansion of the lattice with increasing dose. Of course, also differences in the radiation resistance of the [V–O₄] and [P–O₄] tetrahedral entities may play a role.

4. Conclusions

Two americium vanadates were produced and characterized in terms of crystal structure, cation oxidation states, and chemical, thermal, and radiation stability. The XRD and XANES measurements demonstrate that americium stays in the oxidation state III in the material whatever the redox condition, while vanadium shows the oxidation state III in the perovskite-like AmVO₃ or V in the zircon-like AmVO₄.

After an accumulation of a dose of 10¹⁸ α/g, the close-packed AmVO₃ shows a volume expansion of 4.2% while the low density oxyanionic AmVO₄ contracts with 1.8% in volume (in contrast with other literature reports²⁰). With a fast amorphization behavior, large volume variation, and interchangeable structure as a function of the presence of the oxygen atmosphere and temperature, americium vanadates do not appear to be good candidates as Am forms for radioisotope power systems. Moreover, ²³⁷Np (the decay product of ²⁴¹Am) tends to segregate as a fluorite fcc secondary phase, creating interfaces and inducing stress in the material. Comparing the existing data on Am-containing ceramics with fluorite,³ pyrochlore,³⁴ zircon, monazite,⁵ and perovskite⁴ structures, the (Am,U)O₂^2,3 fluorite solid solution seems currently to be the most suitable form for space applications.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c00251.

• Thermogravimetric behavior of AmVO₃ and AmVO₄; overview of the AmVO₃ and AmVO₄ specimens measured by SEM and EDX; XRD diffraction pattern of AmVO₃ after TGA measurement; Np L₃ XANES spectra of the AmVO₃ and AmVO₄; discussion on the difference in the experimental energy resolution leading to an energy shift of the main maxima of the An M_4,5 edge HR-XANES spectra; discussion on the neptunium dioxide residual phase from XRD data (PDF)

The authors declare no competing financial interest.

Notes

J.-F.V.: Conceptualization, formal analysis, investigation, methodology, validation, and writing (original draft; review and editing); T.W.: Investigation and writing (review and editing); N.P.: Formal analysis, investigation, methodology, validation, and writing (original draft; review and editing); T.V.: Formal analysis, funding acquisition, project administration, validation, and writing (review and editing); J.-Y.C. and D.B.: Investigation; D.F.: Project administration, resources, and writing (review and editing); R.J.M.K.: Project administration, resources, supervision, and writing (review and editing); K.P.: Conceptualization, investigation, methodology, supervision, and writing (original draft; review and editing).