ZnGa(SeO₃)₂F: a UV transparent birefringent crystal explored in M(II)–IIIA–selenite–F systems

Abstract

In this work, we present a targeted design strategy that combines Group IIIA metal cations and fluorine anions to simultaneously enhance the band gap and birefringence in selenite-based systems. Three new ultraviolet (UV) transparent selenite fluorides, SrAl(SeO₃)F₃ (1), BaAl(SeO₃)F₃(H₂O)_0.25 (2), and ZnGa(SeO₃)₂F (3), were successfully synthesized under mild hydrothermal conditions. Remarkably, all three compounds exhibit wide band gaps as well as large birefringence. In particular, ZnGa(SeO₃)₂F (3) demonstrates a pronounced birefringence value of 0.176 at 546 nm, representing the highest birefringence reported among all known selenites with a band gap above 5.0 eV. Theoretical investigations indicate that the strong optical anisotropy is predominantly attributed to the stereochemically active lone-pair electrons of Se(IV) cations. The [Zn₂O₈] dimers and the infinite [GaO₄F]_∞ chains in ZnGa(SeO₃)₂F (3) crucially facilitate the optimal spatial arrangement of [SeO₃] groups. This study expands the structural diversity of selenite fluorides and provides an effective pathway for designing high-performance birefringent crystals suitable for UV applications.

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Introduction

Birefringent crystals, as vital optical components, are indispensable in modulators, sensors, and isolators, finding extensive applications across commercial, industrial, and military sectors.^1–7 Although several established materials, including α-BaB₂O₄,⁸ YVO₄,⁹ CaCO₃,¹⁰ and MgF₂,¹¹ have been commercialized, their performance in high-end optical systems is constrained by inherent limitations. These include the phase transition and facile deliquescence exhibited by α-BaB₂O₄. Additionally, there are challenges in growing sizable single crystals of YVO₄ and CaCO₃. MgF₂ has notably low birefringence, and YVO₄ has limited transparency for wavelengths below 400 nm.^12–14 Concurrently, the ongoing miniaturization of short-wavelength optical devices imposes an escalating demand for crystals that possess a wide band gap and large birefringence.¹⁵ However, achieving this combination is fundamentally challenging due to the inherent trade-off between these two properties, rendering high-performance candidates exceedingly scarce.¹⁶ Therefore, overcoming these material limitations and strategically balancing the band gap and birefringence constitute critical and urgent objectives in the pursuit of next-generation birefringent materials.^17–19

The presence of stereochemically active lone-pair electrons in metal selenites gives rise to pronounced optical anisotropy, rendering these compounds suitable for birefringent applications. To date, numerous metal selenite crystals have been reported. These include compounds containing cations with stereochemically active lone pairs, such as Pb₂(SeO₃)(SiF₆) (0.161@532 nm, 4.40 eV),²⁰ Pb₂Bi(SeO₃)₂Cl₃ (0.186@1064 nm, 3.45 eV),²¹ and Rb₂Bi₂(SeO₃)₃F₂ (0.105@546 nm, 3.72 eV).²² Others include d⁰ transition metal ions characterized by their tendency toward second-order Jahn–Teller distortion, exemplified by Gd₂F₂(OH₂)(MoO₃)₂(SeO₃)₂ (0.143@1064 nm, 3.15 eV),²³ Bi₄TiO₂F₄(SeO₃)₄ (0.190@1064 nm, 3.58 eV),²⁴ and Pb₂(V₂O₄F)(VO₂)(SeO₃)₃ (0.105@1064 nm, 2.35 eV).²⁵ A further category involves d¹⁰ transition metals characterized by high polarizability and distortability, such as Hg₂(SeO₃)(SO₄) (0.133@532 nm, 3.58 eV),²⁶ Hg₃(SeO₃)₂(SO₄) (0.118@546 nm, 4.70 eV),²⁷ Rb₂Hg₂(SeO₃)₃ (0.055@546 nm, 3.60 eV),²⁸ Pb₂Cd(SeO₃)₂Cl₂ (0.093@1064 nm, 4.10 eV),²⁹ and Pb₂Cd(SeO₃)₂Br₂ (0.116@1064 nm, 3.90 eV).²⁹ However, a prevalent limitation among these compounds is their band gap (<5.0 eV), which significantly restricts their application in the UV spectral region. To address this issue, researchers have introduced ions conducive to widening the band gap within the selenite system, such as alkali and alkaline earth metals. This strategy has successfully yielded a series of wide-band gap selenites, including Na₈(SeO₃)(SO₄)₃ (5.69 eV, 0.038@1064 nm),³⁰ Na₂(H₂SeO₃)(SO₄) (5.04 eV, 0.082@1064 nm),³⁰ and NaGa₃(HSeO₃)₆(SeO₃)₂ (5.20 eV, 0.003@1064 nm).¹⁶ Nevertheless, although their band gaps exceed 5.0 eV, the birefringence values of these compounds exhibit a notable decrease, mainly due to the limited contribution of these cations to the anisotropy of polarizability.

Based on extensive literature surveys, fluoride ions (F⁻), which belong to Group VIIA, possess high electronegativity and strong UV transparency.^31–33 The introduction of fluoride ions has been established as one effective strategy for band gap enlargement, as demonstrated by A(GaF₂)₃(SeO₃)₂ (A = K and NH₄),³⁴ demonstrating wide band gaps of 5.62 and 5.77 eV, respectively. Meanwhile, the introduction of Group IIIA metals, such as Al³⁺ and Ga³⁺, does not compromise the band gap, because these metals lack d–d electronic transitions, which could narrow the band gap.^35–38 So far, fluorinated Group IIIA selenites have been predominantly reported in alkali metal and Pb²⁺-based systems, whereas fluorinated Group IIIA selenites incorporating divalent alkaline earth or transition metal cations remain unexplored. Similar to alkali metals, alkaline earth cations possess no d–d and f–f electronic transitions and exhibit strong ionic character, which effectively widens the band gap of their corresponding compounds.^39,40 Furthermore, d¹⁰ transition metal cations (e.g., Zn²⁺ and Hg²⁺), with their high polarization and deformability, are promising for producing materials with large optical birefringence.^41,42 With the aim of developing materials that combine wide band gaps and pronounced birefringence, our research targeted gallium/aluminum selenite fluoride systems. Our work in the M^II–Ga^III/Al^III–Se^IV–O–F systems culminated in the discovery of three novel selenite birefringent materials: SrAl(SeO₃)F₃ (1), BaAl(SeO₃)F₃(H₂O)_0.25 (2), and ZnGa(SeO₃)₂F (3). All three synthesized compounds feature band gaps greater than 5.0 eV. SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 display birefringence values of 0.059 and 0.082 at 546 nm, respectively, whereas ZnGa(SeO₃)₂F exhibits a substantially higher birefringence of 0.176 at the same wavelength. In this work, we present an in-depth analysis of their synthesis procedures, crystal structures, thermal stabilities, and optical properties.

Experimental section

Reagents

SeO₂ (Adamas-beta, 99.99%), Al₂O₃ (Aladdin, 99.9%), Ga₂O₃ (Aladdin, 99.9%), ZnF₂·4H₂O (Adamas, 98%+), SrF₂ (Adamas, 99.9%), BaF₂ (Adamas, 99.9%), and HF (40%, AR) were obtained from commercial sources and used without further purification.

Syntheses

The three compounds were synthesized via a mild hydrothermal reaction. The chemical ratios for each compound are as follows: for SrAl(SeO₃)F₃ (1), the reactants included SeO₂ (0.222 mg, 2 mmol), Al₂O₃ (0.102 mg, 1 mmol), SrF₂ (0.126 mg, 1 mmol), hydrofluoric acid (0.25 mL) and 2.5 mL of deionized water. For BaAl(SeO₃)F₃·(H₂O)_0.25 (2), the reactants included SeO₂ (0.222 mg, 2 mmol), Al₂O₃ (0.102 mg, 1 mmol), BaF₂ (0.175 mg, 1 mmol), hydrofluoric acid (0.25 mL) and 2.5 mL of deionized water. For ZnGa(SeO₃)₂F (3), the reactants included SeO₂ (0.222 mg, 2 mmol), Ga₂O₃ (0.187 mg, 1 mmol), ZnF₂·4H₂O (0.175 mg, 1 mmol), hydrofluoric acid (0.25 mL) and 2.5 mL of deionized water. These mixtures were then sealed in a Teflon liner with a volume of 23 mL and heated at 220 °C for a duration of 4 days. Following this, they were cooled to room temperature at a rate of 3 °C h⁻¹. The resulting products were isolated through vacuum filtration, washed with alcohol, and dried at ambient temperature. Elemental distribution maps provided definitive evidence for the chemical composition of each compound (Fig. 1). As shown in Fig. S2, their purity was verified through X-ray diffraction (XRD) studies.

Fig. 1 SEM images of SrAl(SeO₃)F₃ (a), BaAl(SeO₃)F₃(H₂O)_0.25 (b), and ZnGa(SeO₃)₂F (c) and their elemental distribution maps.

Single-crystal structure determination

Single-crystal XRD data of SrAl(SeO₃)F₃, BaAl(SeO₃)F₃(H₂O)_0.25, and ZnGa(SeO₃)₂F were collected using a Rigaku Oxford Diffraction SuperNova CCD diffractometer on a 293 K (Mo) X-ray source (λ = 0.71073 Å). Data reduction and cell refinement were executed using CrysAlisPro. The structure was elucidated through direct methods and subsequently refined via full-matrix least squares fitting on F², utilizing the OLEX 2-1.5 and SHELXL-2017 crystallographic software packages.^43,44 All atoms underwent refinement with anisotropic thermal parameters. The structural data were further validated using PLATON, revealing no evidence of higher symmetry.^45,46 The detailed crystallographic data of the three structures are listed in Table 1, and some selected atomic bond lengths and bond valences are reported in Table S2.

Table 1 Crystal data and structure refinement for SrAl(SeO₃)F₃ (1), BaAl(SeO₃)F₃(H₂O)_0.25 (2) and ZnGa(SeO₃)₂F (3)

a R1 = ∑||Fo| − |Fo||/∑|Fo|, wR2 = [w(Fo^2 − Fc^2)^2]/[wFo^4]^1/2R1.
Empirical formula	SrAl(SeO3)F3	BaAl(SeO3)F3(H2O)0.25	ZnGa(SeO3)2F
Formula weight	298.56	352.78	408.01
Temperature (K)	293(2)	293(2)	293(2)
Crystal system	Hexagonal	Tetragonal	Monoclinic
Space group	P63/m	P4/n	P21/c
a (Å)	13.2436(8)	14.5731(4)	7.3482(5)
b (Å)	13.2436(8)	14.5731(4)	12.3498(8)
c (Å)	5.1816(4)	5.0773(3)	7.7071(4)
α (°)	90	90	90
β (°)	90	90	121.237(5)
γ (°)	120	90	90
Volume (Å^3)	787.06(11)	1078.29(9)	598.02(7)
Z	6	8	4
Dc (g cm^−3)	3.779	4.346	4.532
μ (mm^−1)	17.374	14.281	20.686
λ (Mo Kα) (Å)	0.71073	0.71073	0.71073
GOF on F^2	1.106	1.165	1.245
R1, wR2 [I > 2σ(I)]^a	0.0238, 0.0554	0.0161, 0.0327	0.0407, 0.0928
R1, wR2 (all data)^a	0.0255, 0.0567	0.0203, 0.0343	0.0445, 0.0942
CCDC	2500271	2500272	2500273

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Powder X-ray diffraction

Powder XRD patterns of the three compounds were collected on a MiniFlex 600 powder X-ray diffractometer using Cu Kα radiation (λ = 1.54186 Å) at room temperature in the angular range of 2θ = 10–70° with a scan step size of 0.02°.

Energy-dispersive X-ray spectroscopy

Elemental analysis was carried out with the aid of a field-emission scanning electron microscope (FESEM, JSM6700F) equipped with an energy dispersive X-ray spectroscope (EDS, Oxford INCA).

Spectroscopic measurements

Infrared (IR) spectra were analyzed utilizing a Magna 750 FT-IR spectrometer, employing air as the background. The analysis was conducted within the range of 4000–400 cm⁻¹ and achieved a resolution of 2 cm⁻¹ at room temperature. UV-vis-NIR spectra were acquired within the 2000–200 nm range using a PerkinElmer Lambda 900 spectrophotometer, with BaSO₄ serving as the reference. The reflection spectra were subsequently transformed into absorption spectra through the application of the Kubelka–Munk function. Absorption data were derived from the diffuse reflection data utilizing the Kubelka–Munk function: α/S = (1 − R)²/2R, where α and S denote the absorption coefficient and scattering coefficient, respectively. The band gap value can be determined by extrapolating the absorption edge to the baseline in the α/S versus energy graph.⁴⁷

Thermal analysis

Thermogravimetric analyses (TGA) were carried out using the Netzsch STA 449F3 installation. About 3.0–5.0 mg of samples were placed in alumina crucibles and heated from 20 to 1000 °C at a rate of 15 K min⁻¹ under a N₂ atmosphere.

Results and discussion

Crystal structure descriptions

SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 have been found to crystallize in the centrosymmetric space groups P6₃/m (no. 176) and P4/n (no. 85), respectively. The asymmetric unit of SrAl(SeO₃)F₃ consists of one Sr, one Se, one Al, two F, and two O atoms. Al exhibits an octahedral geometry coordinated by three O and three F atoms, forming [AlF₃O₃] units, where the Al–O bond lengths span 1.851(9) to 1.914(8) Å. Se1 coordinates with three O atoms, forming an asymmetric trigonal pyramidal structure with Se–O bond lengths of 1.671(6) to 1.690(1) Å. Valence calculations suggest Al and Se oxidation states of +3 and +4. The asymmetric unit of BaAl(SeO₃)F₃(H₂O)_0.25 contains one Ba, one Al, one Se, one H, three F, and four O. It was observed that Al in BaAl(SeO₃)F₃(H₂O)_0.25 adopted a six-coordinate [AlF₃O₃] octahedral geometry with Al–O bond lengths ranging from 1.865(2) to 1.950(5) Å. This finding is analogous to that of SrAl(SeO₃)F₃. Se also coordinates with three O atoms, forming an asymmetric trigonal pyramidal structure with Se–O bond lengths ranging from 1.681(6) to 1.697(2) Å. Valence calculations suggest oxidation states of +3 for Al and +4 for Se, as illustrated in Table S2.^48,49

SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 exhibit structural characteristics that are analogous to each other, as shown in Fig. 2. [AlF₃O₃] octahedra and [SeO₃] trigonal pyramids establish connections via shared oxygen atoms, thereby forming one-dimensional (1D) [AlSeF₃O₄]_∞ chains along the c axis (Fig. 2a and d).

Fig. 2 Structural features of SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25: 1D [AlSeF₃O₄]_∞ chains along the c axis (a and d); 1D porous frameworks with hexagonal (Sr) or tetragonal (Ba) pores (b and e); and 3D structures illustrating cation arrangements within the pores (c and f).

These chains are interconnected by oxygen atoms at the extremities of [SeO₃] trigonal pyramids, thereby establishing a 1D porous framework along the c-axis. In SrAl(SeO₃)F₃, this process results in the formation of hexagonal pores, with Sr²⁺ cations occupying the interstitial spaces (Fig. 2c). BaAl(SeO₃)F₃(H₂O)_0.25 features tetragonal pores as well, with Ba²⁺ cations occupying the interstices between these pores (Fig. 2e).

ZnGa(SeO₃)₂F crystallizes in the monoclinic space group P2₁/c. Its asymmetric unit comprises 11 atoms: one Zn, one Ga, two Se, one F, and six O atoms. The Ga atom exhibits an octahedral coordination environment, bonded to four oxygen and two fluorine atoms. The Ga–O bond distances vary from 1.901(9) Å to 1.967(0) Å, while Ga–F bonds range between 1.951(2) Å and 1.953(8) Å. Each Se atom is coordinated by three oxygen atoms, forming a [SeO₃] trigonal pyramidal unit, with the Se–O bond lengths spanning 1.669(4) Å to 1.751(4) Å. Zinc is coordinated by oxygen atoms with Zn–O distances in the range of 1.960(0) Å to 2.137(4) Å. Bond valence sum calculations yield values of 1.98, 3.25, 3.95, and 3.92 for Zn, Ga, Se(1), and Se(2), respectively, consistent with oxidation states of +2 for Zn, +3 for Ga, and +4 for Se (Table S2).^48,49 ZnGa(SeO₃)₂F has a novel 3D network structure. As illustrated in Fig. 3, the six-coordinated [GaO₄F₂] octahedra share F atoms along the c-axis, forming [GaO₄F]_∞ 1D chains (Fig. 3a). These chains are interconnected via [Se(1)O₃] units to create a 2D layer parallel to the ac plane (Fig. 3c). Two [ZnO₅] polyhedra form a [Zn₂O₈] dimer (Fig. 3b). This dimer shares O atoms with [Se(1)O₃] and [Se(2)O₃] units, generating a separate 1D chain extending along the a-axis (Fig. 3d). The final 3D framework arises from the interconnection of the 2D layers by these 1D chains composed of [Zn₂O₈] dimers and [SeO₃] units (Fig. 3e).

Fig. 3 The 1D [GaO₄F]_∞ chain (a), [Zn₂O₈] dimers (b), 2D layer constructed from the chains and Se(1)O₃ units (c), 1D chain based on [Zn₂O₈] dimers and [SeO₃] units (d), and the overall 3D framework of ZnGa(SeO₃)₂F (e).

Thermal analyses

Thermogravimetric analyses of compounds 1–3 were conducted under a nitrogen atmosphere over the temperature range of 20 to 1000 °C. SrAl(SeO₃)F₃, BaAl(SeO₃)F₃(H₂O)_0.25 and ZnGa(SeO₃)₂F demonstrate thermal stability up to 508 °C, 430 °C, and 396 °C, respectively, with mass losses of 37.9%, 32.1%, and 58.1% occurring over the temperature ranges of 508–800 °C, 430–800 °C and 396–900 °C, in the same order. For SrAl(SeO₃)F₃, the subsequent mass loss is attributed to the evolution of one SeO₂, while for BaAl(SeO₃)F₃(H₂O)_0.25, it involves the release of the same species along with water. For ZnGa(SeO₃)₂F, the mass loss in the 396–900 °C range corresponds to the volatilization of one equivalent of SeO₂ and half an equivalent of F₂ (Fig. S3).

IR and UV-vis-NIR spectra

IR spectra of compounds 1–3 were obtained in the range of 400–4000 cm⁻¹. All three compounds show strong absorption in the range of 563–609 cm⁻¹, which is attributed to the absorption of fluorine ions. The prominent absorption peaks at 708–911 cm⁻¹ correspond to the stretching and bending vibrations of Se–O bonds.⁵⁰ Additionally, the peaks at 433–519 cm⁻¹ and 432–492 cm⁻¹ correspond to the stretching vibrations of Al–O and Ga–O bonds, respectively. In particular, the distinct absorption peaks at 3180 cm⁻¹ and 1601 cm⁻¹ are attributed to O–H vibrational modes.⁵¹ All vibrational assignments are well supported by the existing literature (Fig. S4).

As shown in Fig. 4, the UV-vis-NIR diffuse reflectance spectra indicate minima in reflectance for SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 at about 200 nm and for ZnGa(SeO₃)₂F at 213 nm. The optical band gaps of the three compounds are 5.41 eV, 5.50 eV, and 5.01 eV, respectively. These band gap values exceed those of most selenite fluorides reported previously, such as RbGa₃F₆(SeO₃)₂ (3.57 eV),⁵² CsGa₃F₆(SeO₃) (3.65 eV),⁵² LiBa₃Bi₆(SeO₃)₇F₁₁ (3.80 eV),⁵³ and Bi₃(SeO₃)₃(Se₂O₅)F (3.80 eV),⁵⁴ and are comparable to the largest band gaps for inorganic selenites, for instance, NH₄(GaF₂)₃(SeO₃)₂ (5.77 eV)³⁴ and K₄(GaF₂)₃(SeO₃)₂ (5.62 eV),³⁴ which underscores their unique optical properties.

Fig. 4 UV-vis-NIR diffuse-reflectance spectra of SrAl(SeO₃)F₃ (a), BaAl(SeO₃)F₃(H₂O)_0.25 (b), and ZnGa(SeO₃)₂F (c).

Theoretical calculations

Density functional theory (DFT) calculations were performed to explore the electronic structures and linear optical properties of compounds 1–3, aiming to elucidate their structure–property correlations.⁵⁵ The computed band gap values were 5.07, 5.31, and 4.35 eV, respectively (Fig. S5). Consistent with the known tendency of the GGA-PBE functional (where GGA stands for generalized gradient approximation and PBE for Perdew–Burke–Ernzerhof) to underestimate band gaps due to a lower conduction band energy prediction, the calculated values are smaller than the experimental measurements. To compensate for this discrepancy and enhance the accuracy of the optical property simulations, scissor operators of 0.34, 0.19, and 0.66 eV were introduced for SrAl(SeO₃)F₃, BaAl(SeO₃)F₃(H₂O)_0.25 and ZnGa(SeO₃)₂F, respectively.

To further elucidate the electronic structures, total and partial density of states (DOS) calculations were performed for the three compounds, as depicted in Fig. S6. Examination of the states near the Fermi level reveals the atomic orbitals contributing to the band edges. For SrAl(SeO₃)F₃, the valence band maximum is primarily derived from O-2p and Se-4s4p orbitals, and the conduction band minimum mainly arises from Se-4p orbitals. For BaAl(SeO₃)F₃(H₂O)_0.25, the valence band edge predominantly consists of O-2p states, with the conduction band bottom dominated by Se-4p orbitals. In ZnGa(SeO₃)₂F, the valence band maximum features significant contributions from O-2p and Se-4s4p states, while the conduction band minimum is chiefly composed of Se-4p and Zn-4s orbitals. These results indicate that the band gaps of SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 are determined by Se and O atoms, and the band gap of ZnGa(SeO₃)₂F is governed by Se, O and Zn atoms.

Birefringence

The linear optical properties of the three compounds were analyzed through the complex dielectric function ε(ω) = ε₁(ω) + iε₂(ω), revealing distinct refractive indices along the 100, 010, and 001 crystallographic directions. Compounds 1–3 exhibit the following refractive index sequences: n₀₀₁ > n₀₁₀ > n₁₀₀ for SrAl(SeO₃)F₃, n₀₁₀ > n₁₀₀ > n₀₀₁ for BaAl(SeO₃)F₃(H₂O)_0.25, and n₁₀₀ > n₀₁₀ > n₀₀₁ for ZnGa(SeO₃)₂F (Fig. 5). The computed birefringence values at 546 nm are 0.059 and 0.082 for SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25, respectively, decreasing slightly at 1064 nm to 0.053 and 0.075. In contrast, ZnGa(SeO₃)₂F exhibits significantly enhanced birefringence, reaching 0.176 at 546 nm and 0.163 at 1064 nm. It is noteworthy that the birefringence of ZnGa(SeO₃)₂F exceeds those of all reported selenites with wide band gaps (>5.0 eV) to date, as presented in Fig. 6.⁵⁶ The electron density difference map of ZnGa(SeO₃)₂F reveals that the lone pair electrons of Se(IV) exhibit pronounced stereochemical activity (Fig. S7). It can also be observed that the lone pair electrons on the Se atom in the [Se(IV)O₃] group adopt a linear alignment. This structural feature contributes significantly to the large birefringence of ZnGa(SeO₃)₂F. In comparison with BaAl(SeO₃)F₃(H₂O)_0.25 and SrAl(SeO₃)F₃, the two [ZnO₅] polyhedra forming a [Zn₂O₈] dimer and [GaO₄F]_∞ chains in ZnGa(SeO₃)₂F play an essential role in modulating the arrangement of the lone pair-containing [SeO₃] polyhedra.

Fig. 5 Calculated refractive indices and birefringence values of SrAl(SeO₃)F₃ (a), BaAl(SeO₃)F₃(H₂O)_0.25 (b), and ZnGa(SeO₃)₂F (c).

Fig. 6 Reported birefringence and band gaps of selenites.

Conclusions

In summary, three novel selenite fluoride crystals, SrAl(SeO₃)F₃ (1), BaAl(SeO₃)F₃(H₂O)_0.25 (2) and ZnGa(SeO₃)₂F (3), have been successfully synthesized via mild hydrothermal methods. These compounds exhibit unique structural frameworks, including a 1D porous framework in SrAl(SeO₃)F₃ and BaAl(SeO₃)F₃(H₂O)_0.25 and a 3D network in ZnGa(SeO₃)₂F constructed from [GaO₄F]_∞ chains, [Zn₂O₈] dimers, and [SeO₃] units. Remarkably, all three compounds achieve an excellent balance between a wide band gap and large birefringence. Among them, ZnGa(SeO₃)₂F stands out with an ultra-large birefringence (0.176 at 546 nm) and a wide band gap (5.01 eV). Notably, its birefringence is the largest among all reported selenites with a band gap exceeding 5.0 eV. Theoretical calculations reveal that the large birefringence primarily originates from the stereochemically active lone pairs of Se(IV) in the [SeO₃] groups, as well as the specific arrangement modulated by [Zn₂O₈] dimers and [GaO₄F]_∞ chains in ZnGa(SeO₃)₂F. This work not only enriches the family of selenite-based optical crystals but also demonstrates the effectiveness of combining fluorine and Group IIIA metal cations in designing new birefringent materials with both wide band gaps and large optical anisotropy.

Author contributions

Tian-Tian He: validation and writing – original draft; Chun-Li Hu: formal analysis; Ya-Ping Gong: investigation, validation and funding acquisition; Jiang-Gao Mao: supervision and funding acquisition; Fang Kong: conceptualization, supervision, writing – review and editing and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available in the supplementary information (SI). Supplementary information: computational method, important bond distances, bond valences, as-grown crystals, powder XRD, IR spectra, TGA results, band structures, total and partial density of states, and electron density difference map. See DOI: https://doi.org/10.1039/d5qi02415c.

CCDC 2500271–2500273 contain the supplementary crystallographic data for this paper.^57a–c

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22475215), the Natural Science Foundation of Fujian Province (Grant No. 2024J010039 and 2023j01216), the Self-Deployed Key Project of State Key Laboratory of Functional Crystals and Devices (GNJT-2025-ZD01) and the Shandong Provincial Natural Science Foundation (ZR2024QB208).

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23. Y. X. Ma, P. F. Li, C. L. Hu, J. G. Mao and F. Kong, 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, e2304463.
24. Y. P. Gong, C. L. Hu, F. Kong and J. G. Mao, Exploration of New Birefringent Crystals in Bismuth d0 Transition Metal Selenites, Chem. – Eur. J., 2019, 25, 3685–3694.
25. L. Lin, X. Jiang, C. Wu, Z. Lin, Z. Huang, M. G. Humphrey and C. Zhang, First chiral fluorinated lead vanadate selenite Pb₂(V₂O₄F)(VO₂)(SeO₃)₃ with five asymmetric motifs and large optical properties, Dalton Trans., 2021, 50, 7238–7245.
26. P. F. Li, C. L. Hu, F. Kong and J. G. Mao, Hg₂(SeO₃)(SO₄): the first sulfate selenite with large birefringence explored from d10 transition metal compounds, Mater. Chem. Front., 2022, 6, 3567–3576.
27. J. Ren, Y. Chen, L. Ren, Y. Zhou, X. Dong, D. Gao, L. Huang, L. Cao and N. Ye, Hg₃(SeO₃)₂(SO₄): A UV Nonlinear Optical Mercury Selenite Sulfate Constructed by Neat [Hg₆O₈(SeO₃)₄]_∞ Layers and SO₄ Tetrahedra, Inorg. Chem., 2023, 62, 9130–9138.
28. J. Ren, H. Cui, L. Cheng, Y. Zhou, X. Dong, D. Gao, L. Huang, L. Cao and N. Ye, A₂Hg_x(SeO₃)_y (A = K, Rb, Cs): Three Alkali Metal Mercury Selenites Featuring Unique 1D [HgO_m(SeO₃)_n]_∞ Chains, Inorg. Chem., 2023, 62, 21173–21180.
29. Y. P. Gong, C. L. Hu, Y. X. Ma, J. G. Mao and F. Kong, Pb₂Cd(SeO₃)₂X₂ (X = Cl and Br): two halogenated selenites with phase matchable second harmonic generation, Inorg. Chem. Front., 2019, 6, 3133–3139.
30. X. W. Zhang, Z. X. Wang, C. L. Hu, Y. F. Li, J. G. Mao and F. Kong, UV-Transparent SHG Material Explored in an Alkali Metal Sulfate Selenite System, Inorg. Chem., 2024, 63, 6067–6074.
31. H. Qiu, F. Li, C. Jin, Z. Yang, J. Li, S. Pan and M. Mutailipu, Fluorination Strategy Towards Symmetry Breaking of Boron-centered Tetrahedron for Poly-fluorinated Optical Crystals, Angew. Chem., Int. Ed., 2024, 63, e202316194.
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33. M. Yan, R. L. Tang, W. D. Yao, W. Liu and S. P. Guo, Exploring a new short-wavelength nonlinear optical fluoride material featuring unprecedented polar cis-[Zr₆F₃₄]¹⁰⁻ clusters, Chem. Sci., 2024, 15, 2883–2888.
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35. P. F. Li, C. L. Hu, F. Kong and J. G. Mao, AAl(Te₄O₁₀) (A = Na, Ag) and K₂Ga₂(HTe₆O₁₆)(HTeO₃): Three Aluminum/Gallium Tellurites with Large Birefringence and Wide Band Gap, Inorg. Chem., 2023, 62, 8494–8499.
36. D. D. Zhou, C. L. Hu, X. W. Zhang, J. G. Mao and F. Kong, Noncentrosymmetric tellurite halides created by a depolymerization strategy: toward strong SHG intensity and wide bandgap, Chem. Sci., 2024, 15, 19920–19927.
37. P. F. Li, F. Kong and J. G. Mao, M₂IIM₃IIIF₃(Te₆F₂O₁₆) (MII = Pb, Ba; MIII = Al, Ga): New mixed anionic tellurites with isolated Te₆ coplanar rings, J. Solid State Chem., 2020, 286, 121288.
38. P. F. Li, C. L. Hu, B. Zhang, J. G. Mao and F. Kong, From HgGa₂(SeO₃)₄ to Hg₂Ga(SeO₃)₂F: The first HgI-based selenite birefringent crystal triggered by linear groups and fluoride ions, Chin. Chem. Lett., 2024, 110588,  DOI:10.1016/j.cclet.2024.110588.
39. M. L. Liang, Y. X. Ma, C. L. Hu, F. Kong and J. G. Mao, Ba(MoO₂F)₂(QO₃)₂ (Q = Se, Te): Partial Fluorination of MoO₆ Octahedra Enabling Two Polar Solids with Strong and Phase Matchable SHG Response, Chem. Mater., 2020, 32, 9688–9695.
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48. D. D. Zhou, C. L. Hu, J. G. Mao and F. Kong, Ag₃In₃(SeO₃)₃(SO₄)F₄: A Sulfate Selenite Nonlinear Optical Material with a Wide Bandgap, Inorg. Chem., 2025, 64, 19956–19960.
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51. F. Kong, T. T. He and J. G. Mao, Potassium Molybdenum Tellurite Crystals: Design, Synthesis, and Second-Order Nonlinear Optical Properties, J. Synth. Cryst., 2025, 54, 1811–1822.
52. C. Wu, X. Jiang, L. Lin, Z. Lin, Z. Huang, M. G. Humphrey and C. Zhang, AGa₃F₆(SeO₃)₂ (A = Rb, Cs): A New Type of Phase-Matchable Hexagonal Tungsten Oxide Material with Strong Second-Harmonic Generation Responses, Chem. Mater., 2020, 32, 6906–6915.
53. S. Shi, C. Lin, D. Zhao, M. Luo, L. Cao, G. Peng and N. Ye, Unexpected aliovalent cation substitution between two NLO materials LiBa₃Bi₆(SeO₃)₇F₁₁ and Ba₃Bi_6.5(SeO₃)₇F_10.5O_0.5, Chem. Commun., 2021, 57, 2982–2985.
54. J. Y. Chung, H. Jo, S. Yeon, H. R. Byun, T. S. You, J. I. Jang and K. M. Ok, Bi₃(SeO₃)₃(Se₂O₅)F: A Polar Bismuth Selenite Fluoride with Polyhedra of Highly Distortive Lone Pair Cations and Strong Second-Harmonic Generation Response, Chem. Mater., 2020, 32, 7318–7326.
55. C. L. Hu, Y. X. Han, Z. Fang and J. G. Mao, Zn₂BS₃Br: An Infrared Nonlinear Optical Material with Significant Dual-Property Enhancements Designed through a Template Grafting Strategy, Chem. Mater., 2023, 35, 2647–2654.
56. Y. Zheng, Q. Li, H. Wu, Z. Hu, J. Wang, Y. Wu and H. Yu, Unprecedented v-shaped [SeO₂E] units stabilized in KM[Se₂O₅]₂[SeO₂] (M = Al and Ga) constructed by nanoconfinement strategy, Innovation Mater., 2025, 100160,  DOI:10.59717/j.xinn-mater.2025.100160.
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