Explorations of highly birefringent materials in the vanadium oxyfluoride–iodate system by fluoride ion modulation

Abstract

Birefringent materials are of great interest because of their ability to manipulate light. The demand for smaller devices has driven the development of new birefringent materials with high levels of birefringences and overall excellent physicochemical properties. Two such materials, Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃), have been successfully synthesized by exploring the VOF polyhedron–iodate system. The compounds exhibit remarkable birefringence values of 0.250 and 0.406 at 550 nm, respectively, which are significantly higher than those of the commercially available birefringent vanadate (YVO₄, 0.204 at 532 nm), and the compound Sr₃F₂(VO₂F₄)(IO₃) has the largest birefringence in the iodate-fluoride system. Sr[VO₂F(IO₃)₂] is composed of polymerized anionic groups, [VO₂F(IO₃)₂]²⁻, while Sr₃F₂(VO₂F₄)(IO₃) is composed of optimally arranged IO₃⁻ and fluorinated VO₂F₄³⁻ functional groups organized by the Sr₃F₂⁴⁺ positively charged structure-oriented templates. The compounds have a wide optical transmission range of 0.28–10.7 μm and 0.43–10.3 μm, respectively. Moreover, they exhibit high thermal stability with values of 366 and 319 °C, respectively. These properties make them suitable for use in the middle-wavelength infrared region. The study demonstrates that hybridizing anionic functional groups and modulating the structure with anions are effective crystal engineering strategies for developing high-performance inorganic optical materials.

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Introduction

Linear optical birefringent crystals are of interest because of their ability to split a ray of light into two perpendicularly polarized components.^1,2 They are critical components of devices such as polarimeters, fiber-optic isolators, and circulators.^3–5 Some commonly used birefringent crystals include α-BaB₂O₄ (0.122 at 532 nm),⁶ CaCO₃ (0.172 at 532 nm),⁷ TiO₂ (0.256 at 546 nm),⁸ YVO₄ (0.204 at 532 nm),⁹ and LiNbO₃ (0.0738 at 1440 nm).^10,11 While natural birefringent crystals such as CaCO₃ and TiO₂ often have geological impurities or mechanical processing problems, and α-BaB₂O₄ and LiNbO₃ have relatively low birefringence (Δn), vanadate YVO₄ crystals have been widely used as birefringent materials owing to their high birefringences and excellent machinability.⁹ The development of new birefringent materials with remarkable properties and excellent overall performance has been driven by the need for device miniaturization.^12–14

The relationship between the optical properties of materials and their structures is significant. Therefore, materials with high birefringences can be designed by utilizing microscopic anisotropic functional units.^15–17 Inorganic compounds show promise due to their optical transmission, mechanical processing, and physicochemical properties. Inorganic anisotropic units can be classified into three categories: (1) cations with stereochemically active lone electron pairs (SCALP) (such as Sn²⁺, Pb²⁺, and Sb³⁺),^18–21 (2) transition-metal polyhedra with second-order Jahn–Teller (SOJT) distortions (such as V⁵⁺ and Nb⁵⁺),²² and (3) planar π-conjugated anions (such as BO₃³⁻, NO₃⁻, and CO₃²⁻).^23–26 Conventional birefringent materials consist of the latter two types of anisotropic units; however, compounds with SCALP have only recently been discovered as birefringent materials, such as SnF₂ (Δn, 0.191 at 546 nm)²⁷ and BaSn₂(PO₄)₂.²⁸ I⁵⁺ containing iodates typically have high birefringences and are emerging as a promising class of birefringent materials.^29–33

The combination of two types of functional groups has proven to be a successful approach to study novel materials that benefit from the advantages of both groups. Such compounds include Pb₂VO₂F₅¹⁸ and M(IO₃)₂(NO₃) (M = Sc, In), etc.^34,35 In our previous work, Pb₂(IO₃)(PO₄),³⁶ an iodate–phosphate with improved birefringence and preserved band gap, was successfully synthesized by incorporating IO₃⁻ groups into phosphates. Fluorine element is considered effective for broadening the optical transmission range and forming structure-oriented templates because it has the highest electronegativity and multiple bridging modes. By introducing iodates into hafnium fluorides, we have synthesized a birefringent material, HfF₂(IO₃)₂. This material exhibits a large birefringence of 0.333 at 550 nm and a wide band gap of 4.11 eV, combining the advantages of iodates and fluorides.³⁷

We explored new birefringent materials in the VOF polyhedron–iodate system to enhance the properties of vanadate YVO₄. Two new birefringent materials were successfully synthesized: Sr[VO₂F(IO₃)₂] with polymerized anionic groups and Sr₃F₂(VO₂F₄)(IO₃) with discrete IO₃⁻ and fluorinated VO₂F₄³⁻ functional groups. The structure was modulated by adjusting the fluorine content. Sr₃F₂(VO₂F₄)(IO₃) displays an ultrahigh birefringence of 0.406 at 550 nm, which is the largest in the iodate-fluoride system and has a wide transmittance range of 0.43 to 10.3 μm. Additionally, it remains stable at temperatures up to 319 °C. This paper presents the synthesis, structural analysis, optical characterization, and theoretical calculations of both compounds.

Experimental methods

Synthesis of Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃)

Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) were synthesized using a conventional hydrothermal method. The starting materials for Sr[VO₂F(IO₃)₂] were SrO (0.0518 g, 0.5 mmol), V₂O₅ (0.3638 g, 2 mmol), HIO₃ (0.7036 g, 4 mmol), HF (0.5 mL), and H₂O (2 mL). The mixture was placed in a Teflon liner (23 mL), sealed in a stainless steel autoclave, heated at 200 °C for 72 hours, and cooled to 35 °C at a rate of 3 °C h⁻¹. Pale yellow crystals of Sr[VO₂F(IO₃)₂] were obtained with a yield of 50% (based on Sr) after washing and drying. Sr[VO₂F(IO₃)₂] is iso-structural to α-Ba[VO₂F(IO₃)₂] previously reported by Hongwei Yu et al. The synthesis of Sr[VO₂F(IO₃)₂] required more HIO₃ and lower temperatures than the synthesis of α-Ba[VO₂F(IO₃)₂] (220 °C).³⁸ Sr₃F₂(VO₂F₄)(IO₃) was synthesized using SrO (0.0518 g, 0.5 mmol), V₂O₅ (0.0909 g, 0.5 mmol), HIO₃ (0.1759 g, 1 mmol), HF (0.5 mL), and H₂O (2 mL) at a reaction temperature of 230 °C. Yellow flake crystals of Sr₃F₂(VO₂F₄)(IO₃) were obtained in a higher yield (60%, based on Sr).

The molar ratios of the F, V, and I elements in the reactants play a critical role in synthesizing Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃). The former requires a low F : V : I molar ratio of about 3 : 1 : 1, while the latter requires a much higher F : V : I molar ratio of about 11 : 1 : 1. Temperature also affects the formation of these compounds, with Sr[VO₂F(IO₃)₂] crystallizing at around 200 °C and Sr₃F₂(VO₂F₄)(IO₃) forming at a higher temperature of 230 °C.

Single crystal X-ray diffraction

Single crystal X-ray diffraction data for the two compounds were collected at 295 K using an Agilent SuperNova dual-wavelength CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å). The data were reduced using the CrysAlisPro software package. Numerical absorption corrections were applied using Gaussian integration over a multifaceted crystal model, along with empirical absorption corrections using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm.³⁹ The structures were determined using the direct method and refined through full-matrix least-squares fitting on F² with SHELXL-2017.⁴⁰ All atoms were refined with anisotropic thermal parameters. The crystal structures were examined for missing symmetry elements using PLATON, and no such elements were found.⁴¹ Crystallographic data and information on structure refinement can be found in Table S1 (ESI†), while selected bond lengths and angles are presented in Tables S2 and S3 (ESI†). Detailed information on the crystal structure of the two compounds (CCDC 2299803 and 2299804).†

Powder X-ray diffraction

Powder X-ray diffraction (PXRD) data were collected using a Rigaku MiniFlex600 diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) over a 2θ range of 10–70° with a scan step width of 0.02°.

Energy-dispersive X-ray spectroscopy

Compositional elemental analyses were performed using a field emission scanning electron microscope (JSM6700F) equipped with an Oxford INCA energy dispersive X-ray spectroscope.

Infrared spectroscopy

Infrared (IR) spectra were measured using a Nicolet Magna 750 Fourier transform infrared spectrometer in the 4000–400 cm⁻¹ spectral region. The samples were prepared as pellets with KBr powder.

Ultraviolet-visible-near-IR diffuse reflectance spectra

Ultraviolet-visible-near-IR (UV-vis-NIR) diffuse reflectance spectra were collected using a PerkinElmer Lambda 950 UV-vis-NIR spectrophotometer. A BaSO₄ powder plate served as a 100% reflectance reference. The diffuse reflectance data were converted to absorption data using the Kubelka–Munk function α/S = (1 − R)²/2R, where α is the absorption coefficient, S is the scattering coefficient, and R is the reflectance.⁴² The band gap values were extrapolated from the absorption edge to zero absorption in the plots of α/S versus energy.

Thermal analysis

A NETZCH STA 449F3 thermal analyzer was used to perform thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The measurements were carried out between 30 and 1000 °C at a heating rate of 10 °C min⁻¹ under a flowing N₂ atmosphere. An Al₂O₃ crucible was used as the sample container, and an empty crucible was used as the reference.

Birefringence measurements

The birefringence of the sample was measured using a polarizing microscope (NIKON ECLIPSE LV100N POL) equipped with a Berek compensator at a wavelength of 550 nm. The retardation (Γ) can be calculated using the formula Γ = |n_e – n_o|T = ΔnT, where Δn is the birefringence and T is the crystal thickness. The retardation can be determined by rotating the compensator positively and negatively.

Computational methods

The electronic and optical properties of the materials were calculated using the CASTEP code and the density functional theory (DFT) with the plane-wave pseudopotential method.^43,44 The generalized gradient approximation Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional was utilized. The core-electron interactions were described using the norm-conserving pseudopotential.⁴⁵ The valence electron configurations of the atoms are Sr 5s² 4p⁶ 3d¹⁰, V 3d³ 4s², I 5s² 5p⁵, O 2s² 2p⁴, and F 2s² 2p⁵. The number of plane-wave basis sets was determined using a cutoff energy of 850 eV. The Brillouin zone was sampled using the Monkhorst–Pack method with k-point sampling grids of 5 × 2 × 2 and 1 × 5 × 2 for Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃), respectively. The default values of the CASTEP code were used for other parameters and convergence criteria.

Results and discussion

Crystal structure

Sr[VO₂F(IO₃)₂] crystallizes in the Pbcn (No. 60) space group. Its asymmetric unit contains one Sr, one V, one I, one F, and four O atoms. The structure of Sr[VO₂F(IO₃)₂] consists of [VO₂F(IO₃)₂]²⁻ polyanions and Sr²⁺ cations. Each I⁵⁺ cation is coordinated by three O²⁻ anions in a trigonal pyramidal geometry. The bond lengths between I and O are within the typical range of 1.799(8) to 1.850(7) Å for metal iodates.^46,47 Each V⁵⁺ cation is observed as a dioxovanadium(V) ion (VO₂⁺) with V–O bond lengths of 1.621(7) Å. Each VO₂⁺ ion is connected to a terminal F⁻ anion and two IO₃⁻ groups, forming a [VO₂F(IO₃)₂]²⁻ polyanion (Fig. 1). The V–O bond lengths range from 1.621(7) to 1.968(7) Å, and the V–F bond length is 1.907(9) Å. The angle between the lone electron pairs of the two iodate groups is 12.06°, and the angle between the lone electron pairs and the a axis is 19.36°, caused by the presence of the V–O–I bridges. The [VO₂F(IO₃)₂]²⁻ polyanions are arranged in parallel along the b axis. The polyanions [VO₂F(IO₃)₂]²⁻ are separated by Sr²⁺ cations, which are connected to them by eight Sr–O and two Sr–F ionic bonds. The bond lengths range from 2.602(8) to 2.819(7) Å (See Fig. S1, ESI†). The bond valence sums of Sr (1.879), V (5.132), I (4.975), F (0.880), and O (1.854–2.299) are reasonable.^48,49

Fig. 1 View of the three-dimensional structure of Sr[VO₂F(IO₃)₂] along the a axis (a) and along the b axis (b).

In the structure of iso-structural α-Ba[VO₂F(IO₃)₂],³⁸ the angle between the lone electron pairs of the two iodate groups is 10.96°, and the angle between the lone electron pairs and the a axis is 18.76°. Because the Ba²⁺ ion has a larger radius than the Sr²⁺ ion, these angles in α-Ba[VO₂F(IO₃)₂] are smaller than those in Sr[VO₂F(IO₃)₂].

Sr₃F₂(VO₂F₄)(IO₃) crystallizes in the C2/c space group (No. 15). The asymmetric unit contains three Sr, one V, one I, six F, and five O atoms, all of which are in general positions. The Sr₃F₂(VO₂F₄)(IO₃) compound contains V⁵⁺ cations that exist as dioxovanadium(V) cations (VO₂⁺). However, unlike in Sr[VO₂F(IO₃)₂], each VO₂⁺ is bonded to four F⁻ anions, forming an octahedrally coordinated and discrete VO₂F₄³⁻ complex anion (Fig. 2). The VO₂F₄³⁻ octahedron has two short V–O vanadyl bonds of 1.603(8) and 1.692(6) Å and four V–F bonds ranging from 1.898(5) to 2.177(6) Å. This results in a strongly off-center distortion towards the specific O–O edge (Δd = 0.95), which is much larger than that of the NbO₆⁷⁻ octahedron (Δd = 0.69) of the LiNbO₃ birefringent crystal. Each I⁵⁺ cation is surrounded by three O²⁻ anions in a typical trigonal pyramidal geometry with normal I–O bond lengths ranging from 1.794(7) to 1.804(6) Å. In comparison to the [VO₂F(IO₃)₂]²⁻ polyanion present in Sr[VO₂F(IO₃)₂], the VO₂F₄³⁻ complex anions with multiple fluorine atoms are positioned separately from the IO₃⁻ groups. This separation eliminates the V–O–I bridge bonds and allows for alignment of the VO₂F₄³⁻ and IO₃⁻ groups along the c axis. The lone electron pairs of the iodate groups are arranged in a straight line, forming an angle of only 3.0° with the c axis. This angle is significantly smaller than the corresponding angles observed in Sr[VO₂F(IO₃)₂].

Fig. 2 Three-dimensional structure of Sr₃F₂(VO₂F₄)(IO₃) viewed along the b axis (a). Diagram of the _∞[Sr₃F₂]⁴⁺ chain (b).

Each F1⁻ or F2⁻ ion forms ionic bonds with four Sr²⁺ cations in the tetrahedral geometry, forming a chain of positively charged Sr₃F₂⁴⁺ units. The Sr1²⁺, Sr2²⁺ and Sr3²⁺ cations form ionic bonds with O²⁻ and F⁻ ions of the VO₂F₄³⁻ and IO₃⁻ groups (Fig. 2), serving as templates for the arrangement of the anionic functional units. The Sr–O bond lengths are 2.581(8)–2.637(6), 2.666(7)–2.882(8), and 2.735(7) Å, respectively; the Sr–F bond lengths are 2.457(6)–2.573(5), 2.425(6)–2.876(6), and 2.464(5)–2.622(6) Å, respectively. The bond valence sums of Sr, I, V, F, and O are 2.054–2.166, 5.202, 4.901, 1.004–1.164, and 1.477–2.272, respectively. These values are consistent with their corresponding atomic valences.^48,49

Thermal analysis

TGA and DSC curves show that Sr[VO₂F(IO₃)₂] is stable at temperatures up to 366 °C (See Fig. S3, ESI†). Each molecule then releases 1 I₂ molecule, 2.25 O₂ molecules, and 0.5 F₂ molecules. The calculated total mass loss (65.4%) is in agreement with the experimental value (65.2%). The TGA plot for Sr₃F₂(VO₂F₄)(IO₃) in Fig. 3 shows a dramatic weight loss between 319 and 500 °C, followed by a small weight loss up to 1000 °C. The calculated mass loss of 0.5 I₂ molecules and 2.5 F₂ molecules released per formula unit (35.0%) is higher than the experimental value (30.1%), which is consistent with the continued loss of F₂ molecules above 1000 °C if the residue is assumed to be a mixture of Sr₂V₂O₇, SrO, and SrF₂.

Fig. 3 TGA and DSC curves of Sr₃F₂(VO₂F₄)(IO₃) under a N₂ atmosphere.

Spectroscopic properties

Since the two compounds have identical chemical compositions, their optical spectra are similar. The IR spectra show high transparency in the 4000–1000 cm⁻¹ (2.5–10 μm) region for both compounds (See Fig. S4, ESI†). For Sr[VO₂F(IO₃)₂], the absorption bands at 970 and 906 cm⁻¹ are attributed to V–O vibrations, while the other bands between 794 and 461 cm⁻¹ are attributed to I−O vibrations. For Sr₃F₂(VO₂F₄)(IO₃), the absorption peaks at 970–861 cm⁻¹ and 807–418 cm⁻¹ are assigned to the characteristic vibrations of V–O and I–O, respectively.

The UV-vis-NIR diffuse reflectance spectra indicate that Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) have absorption cutoff edges at 285 and 431 nm, respectively (See Fig. S6, ESI†). The energy band gaps of Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) are 2.39 and 2.33 eV, respectively, based on the extrapolation of the absorption edge to the baseline. The UV-vis-NIR and IR spectra indicate that Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) (Fig. 4) possess a broad optical transmission range of 0.28–10.7 μm and 0.43–10.3 μm, respectively.

Fig. 4 UV-vis-NIR diffuse reflectance spectrum and IR spectrum (inset) of Sr₃F₂(VO₂F₄)(IO₃).

Birefringence properties

The birefringence of a single crystal of Sr₃F₂(VO₂F₄)(IO₃) was measured using a polarizing microscope with a Berek compensator (Fig. 5). The measurements revealed a retardation (Γ) value of 1.815 μm and a crystal thickness (T) of 6.6 μm. By applying the formula (Γ = ΔnT), the birefringence of Sr₃F₂(VO₂F₄)(IO₃) was measured to be 0.275 at a wavelength of 550 nm.

Fig. 5 Original interference image of a single crystal of Sr₃F₂(VO₂F₄)(IO₃) under orthogonally polarized light (a). The crystal under extinction (b). Thickness of the selected measured single crystal (c).

Theoretical calculation

To analyze the correlation between the optical properties and electronic structures of both compounds, we conducted DFT calculations. The band gaps of Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) were theoretically calculated to be 2.88 and 2.66 eV, respectively, as shown in Fig. S7 (ESI†). The discrepancy between the theoretical and experimental values is due to the inherent limitations of the DFT method.

The optical properties of a material are generally related to the electronic states near the Fermi level. In the case of Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃), the distributions of the total and partial density of states are very similar, as shown in Fig. S8 (ESI†). The valence band is mainly occupied by O 2p and a small amount of I 5s and I 5p nonbonding electronic states, while the conduction band is dominated by unoccupied V 3d, O 2p, and I 2p orbitals. The band gap and optical properties of both compounds are determined by the interactions between I and O and between V and O.⁵⁰

The linear refractive index is calculated as n²(ω) = ε(ω), while the complex dielectric function is expressed as ε(ω) = ε₁(ω) + iε₂(ω). The equation below provides the imaginary component (ε₂) of the dielectric function:¹⁶

The fundamental physical constants ħ, e, m, and V represent Planck's constant, charge, electron mass, and unit cell volume, respectively. The Fermi–Dirac distribution functions for the conduction and valence bands are denoted by f_c and f_v, respectively. The term p_cvⁱ(k) refers to the momentum matrix element transition between the conduction band (c) and valence band (v) energy levels at a specific k point in the Brillouin zones. The real part of the dielectric function is obtained by performing the Kramers–Kronig transformation.⁵¹

The calculations show that both compounds exhibit significant optical anisotropy and are optically negative biaxial crystals. At 550 nm, Sr[VO₂F(IO₃)₂] has a birefringence of 0.250 (See Fig. S9a, ESI†), while Sr₃F₂(VO₂F₄)(IO₃) has a birefringence of 0.406 (Fig. 6). These birefringences exceed those of most inorganic oxides (Table S6, ESI†). Moreover, the birefringence of Sr₃F₂(VO₂F₄)(IO₃) is significantly higher than those of all commercially available birefringent crystals and all iodate-fluorides. (See Fig. S9b, ESI†).

Fig. 6 Calculated frequency-dependent refractive indices of Sr₃F₂(VO₂F₄)(IO₃).

Difference between measured and calculated values of birefringence

The birefringence of a crystal is dependent on the direction of the incident light, resulting in a measured value that is typically lower than the calculated value. In the case of biaxial crystals, the crystal exhibits the calculated birefringence value only when the incident light is perpendicular to the plane of the optic axes. The crystal samples of Sr₃F₂(VO₂F₄)(IO₃) used in this study were in their original growth state. The measured birefringence value of the crystal is lower than the calculated value because the plane of incidence is not the plane of the optic axes.

Relationship between crystal structure and birefringence

The correlation between the functional units and birefringence of the materials was analyzed by conducting theoretical calculations of electron density difference. The electron density difference maps show that the anionic functional units are anisotropic, while the electron distribution of the Sr²⁺ cations is spherical. Therefore, the IO₃⁻ and VO_mF_n functional groups contribute to the significant birefringences of Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃) (Fig. 7).

Fig. 7 Electron density difference maps of [VO₂F(IO₃)₂]²⁻ polyanion (a) and VO₄F⁴⁻ polyhedron (b) for Sr[VO₂F(IO₃)₂], and IO₃⁻ group (c) and VO₂F₄³⁻ octahedron (d) for Sr₃F₂(VO₂F₄)(IO₃).

To qualitatively analyze the contribution of the anionic functional units, we calculated the dipole moment vector of each unit. The equation for calculating the dipole moment vector of a molecule is:

Here, the symbol [small mu, Greek, vector] represents the dipole moment vector, q_i denotes the magnitude of the i^th charge, and r⃑_i represents the position of the i^th charge.

For Sr[VO₂F(IO₃)₂], the IO₃⁻ and VO₄F⁴⁻ groups have dipole moments of 14.271 D and 0.178 D, respectively (Table S4, ESI†). These values are slightly higher than the dipole moments of the IO₃⁻ (13.757 D) and VO₄F⁴⁻ (0.0374 D) groups in the compound α-Ba[VO₂F(IO₃)₂].³⁸ In Sr₃F₂(VO₂F₄)(IO₃), the IO₃⁻ and VO₂F₄³⁻ groups have dipole moments of 14.178 D and 9.714 D, respectively (Table S5, ESI†). The dipole moments of these IO₃⁻ groups are comparable to those of other iodates. The dipole moment of the VO₂F₄³⁻ octahedron (9.714 D) in Sr₃F₂(VO₂F₄)(IO₃) is significantly larger than that of the VO₄F⁴⁻ polyhedron in Sr[VO₂F(IO₃)₂] (0.178 D) and most d⁰-TM polyhedra, such as the VO₅F⁶⁻ octahedron in CsVO₂F(IO₃) (6.92 D, 0.04 at 2050 nm),⁵² the VO₅⁵⁻ polyhedron in NaVO₂(IO₃)₂(H₂O) (1.94D, 0.15 at 1064 nm),⁵³ the VO₄F₂⁵⁻ octahedron and the VO₃F₂³⁻ octahedron in K₃V₂O₃F₄(IO₃)₃ (9.10D, 3.92D, 0.158 at 2050 nm),⁵⁴ the VO₄³⁻ polyhedron in Zn₂(VO₄)(IO₃) (0.16 D, 0.180 at 1064 nm),⁵⁵ the MoO₆⁶⁻ octahedron in α/β-BaTeMo₂O₉ (7.52 D, 6.05 D),⁵⁶ and the ZrO₂F₆⁶⁻ polyhedron in CsZrF₄(IO₃) (0.93 D, 0.20 at 1064 nm).⁵⁷ It is evident that non-planar functional groups with large dipole moments contribute to the high birefringence of the material.

Birefringence also depends on the spatial density and arrangement of the functional units. In Sr[VO₂F(IO₃)₂], the density of IO₃⁻ is 1.04 × 10⁻² Å⁻³, while in Sr₃F₂(VO₂F₄)(IO₃), the density of IO₃⁻ plus VO₂F₄³⁻ is smaller, 0.86 × 10⁻² Å⁻³. Therefore, the arrangement of functional units is more important for birefringence. The degree of co-linearity between the units and the angle in relation to the principal refractive axis can be used for evaluation. The smaller the angle, the greater the birefringence. The dipole moment of SCALP IO₃⁻ is oriented opposite to its lone electron pair. In Sr[VO₂F(IO₃)₂], the angle between the lone pairs of the two IO₃⁻ groups is 12.06°, while the angle between the lone electron pairs and the principal axis with the smallest refractive (a axis) is 19.36° (Fig. 1b). The two angles in the α-Ba[VO₂F(IO₃)₂] are smaller than those in Sr[VO₂F(IO₃)₂], so the birefringence of α-Ba[VO₂F(IO₃)₂] (0.278 at 550 nm) is larger than that of Sr[VO₂F(IO₃)₂], although the dipole moment of the functional groups of α-Ba[VO₂F(IO₃)₂] is slightly smaller than that of Sr[VO₂F(IO₃)₂]. In contrast, the IO₃⁻ groups in Sr₃F₂(VO₂F₄)(IO₃) have their lone electron pairs arranged in a straight line, with an angle of only 3.0° between the lone electron pairs and the crystallographic c axis. Therefore, the IO₃⁻ groups in Sr₃F₂(VO₂F₄)(IO₃) are much ordered than in Sr[VO₂F(IO₃)₂] and most vanadium–iodates, such as CsVO₂F(IO₃) (86.8°, 0.04 at 2050 nm),⁵² Cs₂VOF₄(IO₂F₂)⁵⁸ (89.9°, 0.088 at 2050 nm) and α-Ba₂[VO₂F₂(IO₃)₂]IO₃ (30.7°, 36.2° and 33.1°, 0.20 at 2050 nm),³⁸ resulting in a greater contributions to the birefringence.

The effect of the arrangement of the SOJT VO_mF_n polyhedron on birefringence becomes more complicated because these units have more than five polarized bonds. It is widely accepted that birefringence is at its maximum when the dipole moment of the polyhedron is parallel to the principal axis with the highest refractive index. In Sr[VO₂F(IO₃)₂], the dipole moments of the VO₄F⁴⁻ polyhedrons are perpendicular to the principal axis with the highest refractive index, so VO₄F⁴⁻ units do not contribute to the birefringence of Sr[VO₂F(IO₃)₂].

Sr₃F₂(VO₂F₄)(IO₃) crystallizes in the monoclinic space group. The crystallographic b axis coincides with one of the three refractive principal axes, while the other two are situated in the ac plane at a slight angle to the original orthogonal coordinate axes and the crystallographic axes. The effect of the functional units on birefringence was assessed by measuring the difference between the dipole moment components (ΔM) in the directions of the highest and lowest refractive indices. The results presented in Tables S4 and S5 (ESI†) indicate that VO₂F₄³⁻ contributes to the birefringence, while VO₄F⁴⁻ does not, with ΔM values of 0 and 1.347 D, respectively.

In summary, by using more fluorine anions to break the V–O–I bonds in the polymerized anions in Sr[VO₂F(IO₃)₂], the discrete IO₃⁻ groups in Sr₃F₂(VO₂F₄)(IO₃) are optimally aligned, and the anisotropy of the fluorinated VO₂F₄³⁻ polyhedra is much larger than that of the VO₄F⁴⁻ polyhedra, resulting in a higher birefringence of Sr₃F₂(VO₂F₄)(IO₃).

Conclusion

In conclusion, we have successfully synthesized two new VOF polyhedron–iodate, Sr[VO₂F(IO₃)₂] and Sr₃F₂(VO₂F₄)(IO₃). The two compounds have high birefringence values of 0.250 and 0.406 at 550 nm, as well as wide transmission ranges and high thermal stability, superior to the commercially available birefringent yttrium vanadate (YVO₄), and the compound Sr₃F₂(VO₂F₄)(IO₃) has the largest birefringence in the iodate-fluoride system. The birefringence of Sr₃F₂(VO₂F₄)(IO₃) is 1.6 times higher than that of Sr[VO₂F(IO₃)₂], which is attributed to the fact that the higher concentration of fluorine separates the IO₃⁻ and VO₂F₄³⁻ functional units and creates a structural template (Sr₃F₂⁴⁺) that favors the arrangement of the functional groups. The contribution of non-planar functional units to birefringence can be evaluated by qualitatively calculating their dipole moments and the difference in components along the principal axes of the highest and lowest refractive indices. We continue to investigate birefringent materials using functional unit hybridization and structure modulation.

Disclosures

Hydrofluoric acid is a highly corrosive substance that requires careful handling in a fume hood.

Author contributions

Y. Huang synthesized the compounds and performed most experiments. Y. Huang and X. Y. Zhang performed the optical theoretical calculations. S. G. Zhao performed birefringence tests. Y. Huang, B. P. Yang, and J. G. Mao analyzed all the physical tests and wrote the manuscript. All authors provided input on the manuscript.

Conflicts of interest

There is no conflict of interest to report.

Acknowledgements

This research was made possible as a result of a generous grant from the National Natural Science Foundation of China (Grant Number 21975256 and 22031009).

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Footnote

† Electronic supplementary information (ESI) available: Supporting Information is available and includes additional crystallographic data and local dipole moments (Tables S1–S5), Fig. S1–S11, and cif files of crystal structures. CCDC 2299803 and 2299804. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc01200c

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