Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions

Abstract

It has historically been exceedingly challenging to create physically and chemically stable lanthanide compounds with strong second harmonic generation (SHG) due to their strong preference to central symmetry. In this work, five new non‐centrosymmetric lanthanide selenites, namely, Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm, Eu, Gd, Tb and Dy), are achieved by partial fluorination of the lanthanide oxygen polyhedron. An HF corrosion resistant supercritical hydrothermal method is developed, which is a facile and universal method for HF corrosion and high‐temperature high‐pressure environment. The title compounds displayed a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln₂F₂O₁₂(OH₂) dimers. Their powder SHG responses showed a large difference, ranging from 1.0 to 9.0 × KH₂PO₄ (KDP) at 1064 nm. The half‐filled Gd compound exhibited very strong SHG efficiency of up to 1.2 × KTP (KTiOPO₄) at 2050 nm. Compounds Tb and Gd are the first lanthanide selenites with SHG intensity reaching KTP level, which is very rare in this system. Furthermore, these compounds can also possess excellent physicochemical stability and strong luminescence emission, indicating that they are promising multifunctional nonlinear optical materials. This work offered an effective way for design and synthesis of multifunctional and high‐performant nonlinear optical materials.

A general corrosion resistant supercritical reaction method was proposed. By partial fluorination strategy, new and polar rare earth selenites are first achieved in Ln‐Mo‐F‐SeO₃ system. They can display strong SHG and luminescent responses simultaneously. Specifically, Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ features a large and phase matchable SHG intensity of 1.2 × KTP at 2050 nm.

1. Introduction

Nonlinear optical (NLO) crystals, such as second‐harmonic generation (SHG) materials, are one of the key components of laser technology due to their irreplaceable role in expanding the available laser wavelengths.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ^{7 ]} After more than half a century of development, many high‐performance SHG materials have been synthesized.^{[ 8} , ⁹ , ^{10 ]} As modern photonic technology continues to advance in miniaturization and integration, there is a growing demand for multifunctional nonlinear optical materials.^{[ 11 ]} Lanthanide elements are the treasure house of multifunctional materials with excellent magnetic, optical, and electrical properties.^{[ 12} , ¹³ , ¹⁴ , ^{15 ]} However, new non‐centrosymmetric (NCS) lanthanide compounds are hard to obtain because the high coordinated lanthanide polyhedrons are preferred to form CS infinite frameworks. As the retrieved result from the Inorganic Crystal Structure Database, the proportion of NCS compounds in lanthanide oxides is ≈12.4%, which is far lower than that in all inorganic compounds (≈18%). As we know, NCS structure is the prerequisite for SHG crystals.^{[ 16 ]}

It is reported that the following cations or groups are beneficial to the formation of NCS structures: i) lone pair cations (Se⁴⁺, Te⁴⁺, I⁵⁺, ect.),^{[ 16} , ¹⁷ , ^{18 ]} ii) d⁰ transition metal (TM) centered octahedra (MoO₆, WO₆, VO₆, etc.),^{[ 19 ]} iii) π‐conjugated groups (BO₃, NO₃, CO₃, ect.),^{[ 20} , ^{21 ]} iv) tetrahedra groups (PO₄, BO₄, GaS₄, etc.),^{[ 22} , ²³ , ^{24 ]} v) d¹⁰ transition metals (Zn²⁺, Cd²⁺, Hg²⁺).^{[ 25 ]} Self‐assembly of the above ions or groups is an effective way to design and synthesize new SHG materials. For example, the first antimony(III) borate SbB₃O₆ was formed by SbO₄, BO_4, and BO₃ groups, exhibiting a strong SHG intensity of ≈3.5 × KDP,^{[ 26 ]} Furthermore, fluorine element with the most electronegativity can improve the comprehensive properties of SHG materials.^{[ 27} , ^{28 ]} Prof. Pan Shilie's group reported a series of NCS fluorooxoborates or borate fluorides that are expected to be the next generation of deep UV NLO materials by introducing fluorine element into classic borate system, such as NH₄B₄O₆F (cutoff edge: 156 nm, SHG response: 3 × KDP)^{[ 29 ]} and Sr₃B₆O₁₁F₂ (cutoff edge: <190 nm, SHG response: 2.5 × KDP).^{[ 30 ]} etc.

Based on the above considerations, we intended to introduce the d⁰ TM of Mo(VI) with the largest distortion degree and the asymmetric selenite groups with lone pair cations into lanthanide oxide system to explore new multifunctional NLO materials. However, the reported lanthanide molybdenum selenium (IV) oxides were all crystalized in CS space group and SHG‐inactive.^{[ 31} , ³² , ^{33 ]} The nonpolar lanthanide polyhedrons in these compounds were interconnected into CS infinite expanded structures, which hindered the effects of the polar groups.

If the infinite lanthanide network was depolymerized into separate units, the polar groups may play a major role in the structural symmetry, resulting the lattice transformation from CS to NCS. To reduce the interaction of the nonpolar units, partial fluorination of the lanthanide polyhedrons strategy was proposed due to the low valence state and weak bridging ability of fluorine ions. Through extensive literature research, we found that no compounds have been reported in Ln‐Mo‐F‐SeO₃ system.

Based on the known lanthanide molybdenum selenites, the molar ratio of Ln/Mo/Se was fixed on unreported 2/2/2 (Figure  1 ).^{[ 31} , ^{34 ]} To achieve the partial fluorination of lanthanide polyhedron, we have tried some regular synthesis methods, like medium/low‐temperature hydrothermal reactions and high‐temperature solid‐state reactions, but all failed. It is reported that the supercritical hydrothermal method is a distinctive synthesis technique that harnesses the exceptional properties of water in its supercritical state to facilitate reactions under conditions of elevated temperature and pressure.^{[ 35} , ³⁶ , ^{37 ]} This approach holds promise for the discovery of novel NCS compounds. However, the reported supercritical hydrothermal methods cannot handle the reaction environments with both strong acids and fluoride ions. To settle this problem, we developed an acid and HF corrosion resistant supercritical hydrothermal method and achieved five new NSC lanthanide selenites, namely, Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm, Eu, Gd, Tb and Dy) (Table  1 ). They are promising multifunctional materials with strong SHG response and fluorescence performance. Herein, we report the syntheses, structures, and optical properties of the first NCS lanthanide molybdenum selenites.

Figure 1.

Schematic diagram of the strategy for design and synthesis of the title compounds.

Table 1.

Crystal data and structural refinements for Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm, Eu, Gd, Tb, and Dy).

Formula	Sm2F2(OH2)(MoO3)2 (SeO3)2	Eu2F2(OH2)(MoO3)2 (SeO3)2	Gd2F2(OH2)(MoO3)2 (SeO3)2	Tb2F2(OH2)(MoO3)2 (SeO3)2	Dy2F2(OH2)(MoO3)2 (SeO3)2
CCDC No.	2213 643	2213 644	2213 645	2213 646	2213 647
fw	898.52	901.74	912.32	915.66	922.82
Space group	Pmn21	Pmn21	Pmn21	Pmn21	Pmn21
a (Å)	7.0766(6)	7.0775(6)	7.0624(5)	7.0633(2)	7.0329(7)
b (Å)	9.1608(8)	9.1439(8)	9.1047(7)	9.0767(3)	9.0184(10)
c (Å)	9.0309(7)	9.0172(7)	8.9963(7)	8.9761(3)	8.9468(9)
V (Å3)	585.45(8)	583.56(8)	578.47(8)	575.47(3)	567.46(10)
Temperature	293 K	293 K	293 K	293 K	293 K
Z	2	2	2	2	2
Dc (g cm−3)	5.097	5.132	5.238	5.284	5.401
µ(MoKα) (mm−1)	18.293	19.037	19.827	20.695	21.692
GOF on F2	1.028	1.069	1.092	1.048	1.031
Flack factor	−0.018(19)	−0.002(19)	0.01(3)	0.008(17)	0.00(2)
R1, wR2 [I>2σ(I)] a)	0.0285, 0.0612	0.0291, 0.0575	0.0415, 0.0917	0.0275, 0.0560	0.0374, 0.0788
R1,wR2 (all data)	0.0296, 0.0620	0.0316, 0.0597	0.0431, 0.0928	0.0295, 0.0573	0.0414, 0.0813

^a) R₁ = ∑||Fo| − |Fc||/∑|Fo|, wR₂ = {∑w[(Fo)² − (Fc)²]²/∑w[(Fo)²]²}R^1/2.

2. Results and Discussion

The single crystals of the title compounds were synthesized by lanthanide oxide, MoO₃, SeO_2, and hydrofluoric acid under supercritical hydrothermal conditions (Figure 1). The reaction temperature and pressure were 380±1 °C and ≈23 Mpa, respectively. The fluorine source of HF is very important in promoting the crystallization of the title compounds. If the less corrosive fluorides, such as NaF or LnF₃, were used, no crystals could be obtained. To prevent the corrosion of quartz tubes by hydrofluoric acid, the reactants were put in a graphite tube with cover, which was then placed in a slightly larger quartz tube. Use flame to groove the upper end of the quartz tube to prevent the cover of the graphite tube from spraying out. Such acid and HF corrosion resistant supercritical hydrothermal method resulted the first polar lanthanide d⁰‐TM selenite compounds of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) (Figures S1 and S2, Supporting Information).

The isomorphic structures feature a novel 3D framework composed of 1D molybdenum selenite chains bridged by Ln₂F₂O₁₂(H₂O) clusters (Figure  2a), which will be illustrated in detail, represented by the Gd compound. The asymmetric unit of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ contains two Gd, one Mo, two Se, eight O, and one F atoms, as well as one H₂O molecule. The Gd atoms are nine‐coordinated with seven oxygen and two fluorine atoms. The Gd─O and Gd─F bond distances are in the ranges of 2.368(15)–2.511(15) Å and 2.317(9)–2.336(9) Å (Table S1, Supporting Information), respectively, which are close to those reported in related compounds.^{[ 38 ]} The Mo(1) atom is six‐coordinated with six oxygen atoms in a distorted octahedron, with the Mo─O bond distances falling in the range of 1.715(11)–2.219(9) Å. The Se(1) and Se(2) cations are in the center of ψ‐SeO₃ tetrahedron with one vertex occupied by the lone pair electrons. The Se─O bond distances range from 1.642(14) to 1.756(10) Å. Bond valence calculations gave values of 3.27, 3.16, 6.06, 3.90, and 3.82 for Gd(1), Gd(2), Mo(1), Se(1), and Se(2), respectively (Table S2, Supporting Information), indicating that they are in the oxidation states of +3, +3, +6, +4 and +4, respectively.

Figure 2.

View of the 3D structure of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ along the a‐axis (a), the Gd₂F2O₁₂(H₂O) dimer formed by Gd(1) and Gd(2) polyhedra (b), the 1D molybdenum selenite chain composed of MoO₆ and SeO₃ groups (c).

Due to the partial fluorination of lanthanide polyhedrons, the connection between lanthanide polyhedra was restricted, which weakened the influence of lanthanide ions on the structural symmetry. Different from the infinite periodic structures formed by lanthanide oxide polyhedra in literatures, the Gd(1) and Gd(2) atoms in Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ are face‐shared into a Gd₂F₂O₁₂(H₂O) dimer by two F⁻ and one O²⁻ ions (Figure 2b). The distorted MoO₆ octahedra were corner‐shared into a molybdenum oxide chain with the SeO₃ groups bridged on it, forming a new molybdenum selenite chain. As shown in Figure 2c, the selenite groups were arranged on the same side of the molybdenum oxide chain. Due to the blocking effect of the asymmetric SeO₃ units, the MoO₆ octahedron was distorted to the edge formed by O(5) and O(6) atoms, which were located away from the “blocking groups” and coordinated with lanthanide cations. The distorted degree of the MoO₆ octahedron was calculated to be 1.0, which belongs to strong distortion. The molybdenum selenite chains were bridged by the Gd₂F₂O₁₂(H₂O) dimers into a 3D framework with pentagonal tunnels along the a‐axis. The coordinated water molecules were located at the odd member ring tunnels.

FTIR spectra for Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) were recorded in the wave number range of 4000–400 cm⁻¹ at room temperature (Figure S3, Supporting Information). It is noteworthy that these materials were barely absorbed in area of 1638–3396 cm⁻¹. Peaks ≈1633–1638 cm⁻¹ and 3396–3410 cm⁻¹ can be attributed to the stretching vibration of O─H and the bending mode of H─O─H. As illustrated in the enlarged illustration, the bands associated with Se─O and Se─O─Se vibrations appear at 500–800 cm⁻¹. The bands occurring at 850–950 cm⁻¹ can be assigned to Mo─O and O─M─O vibrations.

UV–vis–NIR diffuse reflectance spectra studies indicate that the optical band gaps of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ are 3.20, 3.18, 3.15, 3.05, and 3.24 eV for Sm, Eu, Gd, Tb and Dy compounds respectively (Figure S4, Supporting Information). Sm₂F₂(MoO₃)₂(SeO₃)₂(H₂O) is almost transparent in the range of 2500–1700 nm, and eight continuous small absorption peaks appear after 1700 nm at about 1593, 1552, 1516, 1407, 1375, 1255, 1227, and 1091 nm. Eu₂F₂(MoO₃)₂(SeO₃)₂(H₂O) exhibits several distinct absorptions at 2110, 2035, and 1867 nm, and a wide absorption range at 910–581 nm. Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ is almost transparent in the range of 2500–800 nm, except for a small peak ≈1969 nm. The Tb compound gave three small peaks at 1948, 1885, and 1787 nm, respectively. Dy₂F₂(MoO₃)₂(SeO₃)₂(H₂O) gave ten peaks in the range from 1967 to 745 nm, corresponding to 1967, 1705, 1619, 1292, 1102, 909, 811, 794, 758, and 745 nm, respectively. These absorptions can be ascribed to the f‐f or f‐d transitions of the lanthanide cations.

The thermal behaviors of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) were characterized by thermogravimetric analysis (TGA) (Figure S5, Supporting Information). They displayed similar decomposition curves corresponding to the loss of H₂O, SeO₂, and F₂ molecules from 400 to 1000 °C. From the insert table of Figure S5 (Supporting Information) we can find that these compounds underwent incomplete weight loss at 1000 °C except for Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂. To disclose the decomposition temperature of F₂ and H₂O molecules from Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂, thermogravimetry mass spectrometry was performed (Figure  3a). The results showed that the release of H₂O started at about 400 °C and ended at about 600 °C while the loss of F element occurred at about 565 °C and continued to 1000 °C. Although these compounds contain coordination water, they can still be stabilized up to 400 °C, which was proved by the variable temperature X‐ray powder diffractions (Figure 3b). Such stability can be comparable to some of the anhydrous selenite SHG materials with halogen ions (Table S3, Supporting Information). Moreover, Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ can also exhibit excellent air stability, as evidenced by its ability to maintain a well‐preserved crystal morphology even after six months, as confirmed by PXRD analysis (Figure 3b).

Figure 3.

Thermogravimetry mass spectrometry of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (a) and PXRD of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ at different conditions (b).

Powder SHG measurements revealed that the samples of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂(Ln = Sm‐Dy) can display strong SHG signals of about 4.8, 5.0, 5.7, 9.0, and 1.0 × KDP at 1064 nm, respectively, which is much larger than that of Lu₃F(SeO₃)₄ with the largest SHG intensity in reported lanthanide selenites (Figure  4 ). We can find that their SHG intensities are totally different although they are isostructural. It is worth mentioning that the SHG intensities of these compounds have negative correlation with their optical band gaps, with the larger bandgap, the weaker the SHG intensity, which is consistent with the literatures.^{[ 39 ]} However, the largest difference of the bandgap is only 0.19 eV, but the SHG intensity is nine times different. We think such a difference should be caused by the different absorption around the wavelengths of incident and frequency doubling light. Due to the strong f‐f transitions of Dy(III), Dy₂F₂(MoO₃)₂(SeO₃)₂(H₂O) displays the weakest SHG intensity in the five compounds. As to compounds Sm to Tb, their SHG intensity is increased with the increase of the atomic number of the lanthanide, which corresponds to the results reported in 2021.^{[ 40 ]}

Figure 4.

SHG intensity and band gap of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) compared with the reported Lu₃F(SeO₃)₄.^{[ 15 ]}

To avoid the influence of f‐f transition, the semi‐filled Gd compound was chosen for further research. The SHG intensity on different particle size was studied to evaluate whether it can realize phase matching (Figure  5 ). From the inset of Figure 5a, we can find that the SHG intensity is increased with the increase of the particle size until it reaches saturation, which indicates that Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ can realize phase matching at 1064 nm. To explore its performance in near infrared wavelength, we extended the incident wavelength to 2.05 µm. KTP was used as the reference, and its powder SHG intensity is one order of magnitude larger than that of KDP. As Figure 5b shows, the SHG intensity for Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ is ≈1.2 times that of KTP under the radiation of 2.05 µm laser, which is comparable with that of BiFSeO₃.^{[ 41 ]} Furthermore, Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ can also realize phase matching at 2050 nm. So, Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ can be used at visible and near‐IR wavelength bands.

Figure 5.

Oscilloscope traces of the SHG signals of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ at (a) 1064 nm and (b) 2.05 µm. Plots of SHG intensity versus particle size are shown in the insets. KDP and KTP samples serve as the references at 1064 nm and 2.05 µm, respectively.

The DFT method was employed to investigate the underlying structural and electronic factors contributing to the pronounced SHG effect observed in Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Table S4, Supporting Information). It features an indirect bandgap compound with a calculated bandgap of 1.87 eV (Figure S6, Supporting Information), which is much lower than the experimental result of 3.15 eV. So, a scissor of 1.28 eV was used in the following calculations. As the partial density of states (PDOS) of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ shown, Gd demonstrates a significant degree of spin polarization in its states, contrasting with the relatively balanced spin‐up and spin‐down states observed in other atoms (Figure  6 ). The primary composition of the highest valence band is attributed to the nonbonding states of O‐2p while the conduction band bottom is predominantly governed by the unoccupied Mo‐4d and O‐2p states. So, the band gap of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ is determined by Mo and O atoms. Its birefringence was calculated to be 0.143 and 0.133 at 1064 and 2050 nm relatively, which are large enough for the phase‐matching (Figure S7, Supporting Information). The shortest type I phase‐matching SHG wavelength of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ was estimated as 420 nm according to the theoretical refractive index dispersion profiles (Figure S8, Supporting Information).

Figure 6.

Calculated total and partial density of states of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂.

The largest SHG tensor d ₃₁ was calculated to be 12.97 pmV⁻¹, slightly larger than the experimental value of KTP (d ₃₃@1313 nm = 11.1±0.6 pm V⁻¹),^{[ 42 ]} which is consistent with our powder SHG measurements. SHG‐weighed electron density was calculated to represent the SHG effect distributions in Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂. As depicted in Figure  7 , the SHG effects in the valence band predominantly arise from the O‐2p nonbonding orbitals, whereas in the conduction band, the SHG process is primarily influenced by the unoccupied electronic states of Mo‐4d in conjunction with O‐2p states. By considering the overall SHG density in both the valence band and conduction band, the respective SHG contribution percentages were calculated as 56.6% for the MoO₆ octahedra, 24.5% for the SeO₃ group, and 17.9% for the GdO₇F₂ polyhedra. It is evident that the severely distorted MoO₆ octahedra are the major factor to the SHG effect and each building unit has a beneficial impact on the SHG process, leading to the remarkable properties of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ as a promising SHG material.

Figure 7.

SHG density of d ₃₁ in the valence band (a) and conduction band (b) of Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂.

In addition to the frequency‐doubling property, these compounds may also possess good fluorescence and magnetic properties since they contain active lanthanide ions. Due to the coupling of frequency‐doubling and laser may result self‐frequency‐doubling laser crystals, we focused on their luminescent properties. Except for Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂, which is almost transparent in the range of 2500–800 nm, the fluorescence properties of the other four compounds were explored as follows.

Sm₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: As shown in Figure  8a, visible emission spectra have been recorded at 410 nm, 442 nm, and 461 nm under excitation (dotted blue line). Under excitation at 410 nm, the sample emits transitions of ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 corresponding to the emission of Sm³⁺ at around 562 nm, 597 nm, and 644 nm, respectively (red solid line). This result is consistent with the previously reported for Sm³⁺ ions in the red‐orange range of the spectrum.^{[ 43 ]}

Figure 8.

Photoluminescence properties of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ [Ln = Sm (a), Eu (b), Tb (c) and Dy (d)].

Eu₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: Eu³⁺ is a well‐known red phosphor^{[ 44 ]} with a typical strong emission transition ⁵D₀→⁷F₂ at ≈612 nm with the red light emitting. The characteristic ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) transition peaks of Eu³⁺ exist in compound Eu₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ with the luminescence range around 580 nm for ⁵D₀→⁷F₀, 593 nm for ⁵D₀→⁷F₁, 617 nm for ⁵D₀→⁷F₂, 651 nm for ⁵D₀→⁷F₃ and 696 nm for ⁵D₀→⁷F₄ transition (Figure 8b).

Tb₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: Its known to all, Tb³⁺ is one of the ions producing green light.^{[ 45 ]} As shown in Figure 8c, Tb₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ phosphor showed four characteristic Tb³⁺ transitions in the emission spectrum under UV light at 378 nm: ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄ and ⁵D₄→⁷F₃ at ≈490 nm, 544 nm, 594 nm, and 621 nm, respectively. The ⁵D₄→⁷F₅ green emission is the dominant one for this compound.

Dy₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: Dy³⁺ can give different colors for its luminous behavior.^{[ 46 ]} Dy³⁺ can be used to obtain green emission (emission peak ≈480 nm), but it is also possible to use Dy³⁺ ions directly to obtain yellow light from a strong emission peak around 574 nm. For Dy₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Figure 8d), under the excitation wavelength of 389 nm, the characteristic strong peaks were found ≈480 and 573 nm, corresponding to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions, respectively.

3. Conclusion

In summary, the first polar lanthanide d⁰‐TM selenites, namely, Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm, Eu, Gd, Tb and Dy), were achieved by partial fluorination strategy under an acid and HF corrosion resistant supercritical hydrothermal method. Their structures displayed a novel 3D framework consisting of 1D molybdenum selenite chains bridged by Ln₂F₂O₁₂(OH₂) dimers. Compounds Sm, Eu, Tb, and Dy can exhibit strong luminescence in orange, red, green, and yellow regions, respectively. Due to the different characteristics of the lanthanide ions, their powder SHG responses showed large differences, ranging from 1.0 to 9.0 × KDP at 1064 nm. Interestingly, the SHG intensity of compound Sm to Tb increases with the increase of the atomic number of the lanthanide element. A detailed study on the SHG property of the half‐filled Gd compound shows Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ can realize phase matching at both 1064 and 2050 nm and its SHG intensity at 2050 nm can reach to 1.2 × KTP. The strong SHG efficiency was contributed by the synergistic effect of MoO₆, SeO₃ and LnO₇F₂ groups with percentages of 56.6%, 24.5%, and 17.9%, respectively based on the DFT calculations. This work proved that the corrosion resistant supercritical reactions assisted partial fluorination strategy is an effective method to create multifunctional nonlinear optical materials in lanthanide compounds.

4. Experimental Section

All the reagents were obtained from commercial sources and employed without further refinement: Sm₂O₃ (Adamas‐beta, 99.9%), Eu₂O₃ (Adamas‐beta, 99.9%), Gd₂O₃ (Adamas‐beta, 99.9%), Tb₄O₇ (Adamas‐beta, 99.9%), Dy₂O₃ (Adamas‐beta, 99.9%), MoO₃ (Adamas‐beta, 99.5%+), hydrofluoric acid (HF, Adamas‐beta, 40%), and SeO₂ (Adamas‐beta, 99.9%). ( Caution! Hydrofluoric acid is toxic and corrosive! It must be handled with extreme caution and the appropriate protective equipment and training.)

The single crystals of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) were synthesized by Ln₂O₃ (Ln = Sm, Eu, Gd, and Dy) or Tb₄O₇, MoO₃, SeO₂ and hydrofluoric acid under supercritical hydrothermal conditions. A mixture of Ln₂O₃ (0.349 g, 1 mmol for Sm₂O₃, 0.352 g, 1 mmol for Eu₂O₃, 0.363 g, 1 mmol for Gd₂O₃, 0.374 g, 0.5 mmol for Tb₄O₇ and 0.373 g, 1 mmol for Dy₂O₃), MoO₃ (0.288 g, 2.0 mmol), SeO₂ (0.333 g, 3.0 mmol), hydrofluoric acid (0.5 mL) and 4 mL of H₂O were added in a graphite tube with cover, which was then placed in a slightly larger quartz tube. Use the flame to groove above the quartz tube to prevent the cover of the graphite tube from spraying out. Finally, the quartz tubes were put in a high‐temperature and high‐pressure reactor with required amount of water. The hydrothermal autoclave was heated to 380 °C and kept at this temperature for 3 days, then cooled down to room temperature at a rate of 6°C h⁻¹. The products were filtered with deionized water. Light yellow crystals were obtained with yield of about 20% (based on Ln). Their purities were checked by the powder X‐ray diffraction (PXRD), which were consistent well with the calculated patterns based on the crystal structures (Figure S1, Supporting Information). As shown in the elemental distribution maps, elements of Ln, Mo, Se and F are evenly dispersed in the crystals of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) (Figure S2, Supporting Information).

Single‐crystal XRD data for the five compounds were collected on an Agilent Technologies SuperNova dual‐wavelength CCD diffractometer with Mo‐Kα radiation (λ = 0.71073 Å) at 293 K. Data reduction was performed with CrysAlisPro, and absorption corrections based on the multiscan method were applied.^{[ 47 ]} The single crystal structures were determined by the direct methods refined by full‐matrix least‐squares fitting on F² using SHELXL‐97.^{[ 48 ]} All of the atoms were refined with anisotropic thermal parameters and finally converged for F ₀ ² ≥ 2σ(F ₀ ²). The structural data were also checked for possible missing symmetry with the program PLATON, and no higher symmetry was found.^{[ 49 ]} Crystallographic data and structural refinements of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) are listed in Table 1. The selected bond distances are listed in Table S1 (Supporting Information).

Powder X‐ray diffraction (PXRD) patterns were collected on a Rigaku MiniFlex II diffractometer using Cu‐Kα radiation in the angular range of 2θ = 5−65° with a step size of 0.02°.

Microprobe elemental analyses and the elemental distribution maps were measured on a field‐emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy‐dispersive X‐ray spectroscope (EDS, Oxford INCA).

FTIR spectra were carried out on a Magna 750 FT‐IR spectrometer using KBr as the diluent in 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹ at room temperature.

The UV–vis–NIR diffuse reflectance spectra were measured at 200–2500 nm by a PE Lambda 900 UV–vis–NIR spectrophotometer using BaSO₄ as the reference. Absorption data was calculated from the diffuse reflection data by the Kubelka‐Munk function: α/S = (1‐R)²/2R, where α and S represent the absorption coefficient and the scattering coefficient, respectively.^{[ 50 ]}

Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449C instrument with a heating rate of 15 °C min⁻¹ under a nitrogen atmosphere from 30 to 1000 °C.

Thermogravimetric‐Mass spectrometer (TG‐MS) analysis was performed on synchronous thermal analyzer (model STA 449 F5) and mass spectrometer (model QMS403C).

Powder SHG measurements were conducted using a modified method of Kurtz and Perry.^{[ 51 ]} Irradiation laser (λ = 1064 nm and λ = 2.05 µm) is generated by a Nd:YAG solid‐state laser equipped with a Q switch. The Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ pure crystal samples ground into powder were sieved according to seven different particle size ranges (45−53, 53−75, 75−105, 105−150, 150−210, and 210−300 µm). KH₂PO₄ (KDP) and KTiOPO₄ (KTP) samples in the same size ranges were also prepared, which were used as reference. SHG signals oscilloscope traces of Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (Ln = Sm‐Dy) and KDP/KTP samples in the particle size range (150−210 µm) were recorded.

Photoluminescent properties including the emission and excitation spectra in solid state were measured on a FLS920 Edinburgh fluorescence spectrometer.

[Further details of the crystal structure investigation(s) may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein‐Leopoldshafen (Germany), on quoting the CCDC depository numbers 2213643−2213647].

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Ma Y.‐X., Li P.‐F., Hu C.‐L., Mao J.‐G., Kong F., Ln₂F₂(OH₂)(MoO₃)₂(SeO₃)₂: Promising Multifunctional Nonlinear Optical Materials Created by Partial Fluorination Strategy under Corrosion Resistant Supercritical Reactions. Adv. Sci. 2023, 10, 2304463. 10.1002/advs.202304463

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.