NaVSeO₅: synergistic combinations to synthesize excellent birefringent materials

Abstract

Novel NaVSeO₅ birefringent crystals were successfully synthesized using a multifunctional group synergistic strategy, which combines a two-dimensional layered structure consisting of stereochemically active lone pairs [SeO₃] and anomalous [VO₆] polyhedra. Polarization microscopy tests reveal that NaVSeO₅ possesses significant birefringence (0.180@546 nm). Theoretical analysis suggests that this birefringence is attributed to the synergistic effect between highly distorted V–O polyhedra and stereochemically active lone pairs of Se⁴⁺. Based on UV-Vis-NIR diffuse reflectance analysis, the crystal NaVSeO₅ demonstrates a cut-off edge below 336 nm. This study provides valuable insights for future exploration of novel birefringent crystal materials.

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Introduction

Crystals with anisotropic arrangements of units have a broad spectrum of physical features, one of which is birefringence (Δn), a vital and essential optical property for optical materials.^1,2 It is essential to examine artificial crystals’ birefringence in optical functional materials.^3–7 Birefringent crystal materials, a type of linear optical material, are widely used in high-precision scientific research tools, optical communication, and various other scientific and technological disciplines.^8,9 They are also capable of modulating and detecting the polarization state of light.^10,11 In recognition of their extraordinary properties, a variety of birefringent materials have been becoming more and more well-known in particular fields over the past few decades.^12,13

There are some famous birefringent crystals available commercially, such as YVO₄ (0.208@1064 nm),^14,15 CaCO₃ (0.172@532 nm),¹⁶ LiNbO₃ (0.074@546 nm),¹⁷ α-BaB₂O₄ (0.122@546 nm),^18,19 and TiO₂ (0.256@1530 nm).²⁰ However, these materials still have some inherent limitations, like the relatively narrow band transmittance of YVO₄ and TiO₂^21,22 and the impurities of calcite.²³ As for α-BBO, the growing issue of phase transition is energy demanding and leads to the high cost of developing high quality single crystals.^24,25 In other words, existing crystal materials struggle to meet the evolving practical needs of societal development. The creation of birefringent crystals with substantial optical anisotropy for use in optical elements is consequently of tremendous scientific and technological relevance.^26–29

To design and synthesize novel compounds with large birefringence, lots of literature reports have been investigated. V–O anionic groups were first chosen because V–O contributes up to 70% to the birefringence of the commercial material YVO₄.³⁰ Additionally, Sn₂B₅O₉X (X = Cl and Br)^31,32 has enhanced birefringence compared with the isostructural M₂B₅O₉X (M = Ca, Sr, Ba, and Eu, X = Cl, Br, and I). Its birefringence is shown to be mostly attributed to the stereochemically active lone pair (SCALP) cation. So stereochemically active lone pair (SCALP) cations such as Te⁴⁺, Sn²⁺, and Se⁴⁺ could be selected^33–37 to achieve higher birefringence. Furthermore, transition metals having d⁰ electronic configuration (d⁰ TM), such as V⁵⁺, Nb⁵⁺, and Mo⁶⁺, are also favourable to the birefringence due to second-order Jahn–Teller (SOJT) distortions and the distortion degree of the d⁰-TM centered polyhedron.^38–41

Based on these ideas, our research focuses on combining hybrid anionic functional building blocks containing the stereochemically active component Se⁴⁺ and susceptible to SOJT distortion V⁵⁺ octahedra into a single structure. This approach is used to enhance optical anisotropy in the quest for birefringent materials with a short cut-off edge and superior birefringence. After numerous attempts, a novel NaVSeO₅ birefringent material (Δn = 0.180@546 nm) was successfully synthesized. Its synthesis, crystal structure, UV-visible-near-infrared diffuse reflectance spectra, and vibrational spectra are reported in this paper. In addition, the first principles result reveals that its large birefringence originates from the VO radical and the SeO moiety.

Experimental

Reagents

The starting agents Na₂CO₃, (Tianjin BaiShi Chemical Co., Ltd, 99.8%), SeO₂, (Aladdin Chemical Industry Co., Ltd, 99.0%), and V₂O₅, (Beijing Chemical Industry Co., Ltd, 99.0%) were used as received without any further treatment.

Single crystal growth

NaVSeO₅ crystals were obtained for the first time in a sealed environment. The raw materials Na₂CO₃, SeO₂, and V₂O₅ in a molar ratio of 1 : 1 : 9 were placed in a clean and dry quartz tube (Φ 10 mm × 100 mm). The quartz tube was pumped to a high vacuum of 10⁻³ Pa and sealed and then put into a muffle furnace and heated from room temperature to 450 °C for 13 h and held for half a day. Subsequently, it was slowly cooled to 280 °C at 2 °C h⁻¹ and then to 25 °C at a rate of 5 °C h⁻¹. Single crystals of NaVSeO₅ were manually selected after opening the quartz glass tube.

Single crystal X-ray crystallography studies

The small colorless single crystals of NaVSeO₅ of 0.17 × 0.14 × 0.12 mm³ were stuck to a glass fiber to record the crystal structure data using an APEX II CCD X-ray diffractometer equipped with monochromatic Mo–Kα radiation at 296 K. The absorption corrections and data integration were achieved using the SAINT program. The structure was solved by direct methods using the SHELXTL crystallographic program, and the full matrix least-squares techniques were applied to refine the position of all the atoms.^42–46 Finally, we acquired NaVSeO₅ crystal structures with a degree of completeness of more than 99%. Table 1 shows the crystal data and structure refinement of NaVSeO₅. The PLATON program was used to verify the symmetry of the structure.⁴⁷ The atomic coordinates, bond valence sums (BVS), selected bond lengths (Å), and bond angles are shown in Tables S1–S3 in the ESI†.

Table 1 Crystal data and structure refinement for NaVSeO₅

a R 1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 = [∑w(Fo^2 − Fc^2)^2/∑wFo^4]^1/2 for Fo^2 > 2σ(Fo^2).
Empirical formula	NaVSeO5
Formula weight	232.89 g mol^−1
Crystal system	Monoclinic
Space group, Z	C2/c, 8
Unit cell dimensions (Å)	a = 18.234(3)
	b = 3.8381(6), β = 112.421(7)
	c = 12.416(2)
Volume	803.2(2) Å^3
Density (calc.)	3.852 g cm^−3
F(000)	864
θ range for data collection	2.42 to 27.52°
Limiting indices	−23 ≤ h ≤ 23, −4 ≤ k ≤ 4, −16 ≤ l ≤ 16
Reflections collected	6766
Independent reflections	2478 [Rint = 0.0645]
Data/restraints/parameters	910/0/73
Completeness to θ (%)	99.9%
Refinement method	Full-matrix least-squares on F^2
Goodness of fit on F^2	1.066
Final R indices [Fo^2 > 2δ(Fo^2)]^a	R 1 = 0.0317, wR2 = 0.0652
R indices (all data)^a	R 1 = 0.0407, wR2 = 0.0698
The largest diff. peak and hole	0.895 and −1.042 e·Å^−3

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Solid-state synthesis

The polycrystals of the title compounds were synthesized by solid-state reactions of Na₂CO₃ SeO₂, and V₂O₅ in a 1 : 1 : 1 molar ratio. Compound NaVSeO₅ was synthesized by heating a homogeneous mixture of the samples to 120 °C, annealing for 7 h, and then heating the samples to 350 °C and maintaining them for 8 h through multiple intermediate grinding and mixing.

Powder X-ray diffraction

The purity of polycrystalline NaVSeO₅ was confirmed using a Bruker D2 PHASER X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The diffraction data were collected in the range of 5–120° (2θ) with a fixed counting time and a scan step width of 1 s per step and 0.02°, respectively. The diffraction pattern agrees well with the theoretical one, as shown in Fig. 1a.

Fig. 1 (a) Rietveld refinement of the powder X-ray diffraction pattern of compound NaVSeO₅ polycrystals. A full Rietveld refinement was carried out using GSAS II software. (b) EDS maps of Na, V, Se, and O in the compound crystal. Scale bar is 2.5 μm.

Rietveld refinement

The polycrystals were obtained directly by grinding the obtained crystals and the Rietveld refinement⁴⁸ was used to verify the correctness of the solved structure and the purity of the samples (shown in Fig. 1a). The fit curves match the experimental data obtained from the single-crystal CIF data, leading to reasonable R values of W_R = 0.0877 and R_p = 0.0576, respectively, and all observed Bragg reflections are indexed to the C2/c space group, respectively.

EDS analysis

Energy dispersive X-ray spectroscopy (EDS) was carried out on a clean single-crystal surface with the aid of a field emission scanning electron microscope (SUPRA 55VP) equipped with an energy dispersive X-ray spectrometer (Bruker xflash-sdd-5010). Additionally, EDS was conducted on a NaVSeO₅ single crystal, confirming the presence of the corresponding elements within the compound (shown in Fig. 1b).

UV–Vis–NIR diffuse reflectance measurement

To determine the cutoff edge of the title crystal, the UV–Vis–NIR diffuse reflectance spectrum was recorded. The diffuse reflectance spectrum was recorded at room temperature with a Shimadzu SolidSpec-3700DUV spectrophotometer.^49,50

IR spectroscopy

The infrared (IR) spectrum was recorded to specify the coordination of the V and Se atoms. The IR spectrum was recorded on a Shimadzu IRAffinity-1 Fourier transform IR spectrometer in the range of 400–4000 cm⁻¹. The sample was mixed thoroughly with dried KBr.⁵¹

Thermal analyses

Thermal gravimetry (TG) and differential scanning calorimetry (DSC) were carried out to examine the thermal stability of NaVSeO₅ on a simultaneous NETZSCH STA 449 F3 thermal analyzer instrument under a flowing N₂ atmosphere. The sample was placed in a Pt crucible and heated from 40 to 800 °C at a rate of 5 °C min⁻¹.

Birefringence

The refractive index difference of NaVSeO₅ was characterized by using a polarizing microscope equipped (ZEISS Axio Scope. A1) with a Berek compensator. The refractive index difference of the crystal at 546 nm was calculated using the equation R = Δn × d, where R, Δn, and d denote retardation, the refractive index difference, and thickness, respectively. The natural growth plane of the crystals does not always align with a specific optical principal axis, so the measured refractive index difference is smaller than the actual birefringence. In that sense, the birefringence of NaVSeO₅ should be close to or larger than the test value.⁵²

Theoretical computations

In an effort to further explore the factors influencing the optical properties of crystals, the electronic structures and optical properties of NaVSeO₅ were analysed using the CASTEP⁵³ code based on density functional theory (DFT).^54,55 During the calculation, the exchange–correction functional was selected with Perdew–Burke–Ernzerhof (PBE)⁵⁶ in the generalized gradient approximation (GGA). Geometry optimization was performed, and the tolerance of total energy, max. ionic force, max. ionic displacement, and max. stress are set as 5 × 10⁻⁶ eV per atom, 1 × 10⁻¹eV Å⁻¹, 5 × 10⁻⁴ Å and 2 × 10⁻² GPa, respectively. The core electrons are treated using Norm-Conserving Pseudopotentials (NCP),^57,58 and the valence electrons are set as Na 2s² 2p⁶ 3s¹, V 3d³ 4s², Se 4s² 4p⁴, and O 2s² 2p⁴. The plane-wave energy cutoff was set at 830 eV and the separation of the k-point was set as 0.04 Å⁻¹ in the 7 × 7 × 2 Brillouin zone. The other calculation parameters and convergent criteria were set as the default values of the CASTEP code. After obtaining the electronic structure, the refractive indices and birefringence were further calculated using the OptaDOS^59,60 code. The functional basic units’ contribution to the optical response is analysed using the real-space atom-cutting (RSAC) method.⁶¹

Results and discussion

Crystal structure

Analysis of single crystal diffraction data reveals that NaVSeO₅ adopts a monoclinic crystal system with a centrosymmetric space group of C2/c (no. 15) (shown in Table 1). The asymmetric unit of the structure comprises one Na, one V, one Se, and five independent O atoms (shown in Table S1†). The crystal structure of NaVSeO₅ is characterized by 2D layers, which are composed of isolated trigonal pyramid [SeO₃] groups and 1D [VO₄]_∞ infinite chains (shown in Fig. 2c). The V atoms are connected by oxygen atoms, forming [VO₆] distorted octahedra, while the Se atoms are coordinated with three oxygen atoms to create trigonal pyramid [SeO₃] groups (shown in Fig. 2a). In order to accurately represent the distortions of [VO₆] and [SeO₃], the distortions were evaluated using the Baur method implemented in the VESTA code.^62,63 The distortions (Δd) are 0.119 and 0.027, respectively. Notably, the bond lengths and angles within the NaVSeO₅ structure closely resemble the structures found in previously reported vanadate-selenites (shown in Table S4†).^64,65 Initially, these [VO₆] octahedra have consistent deformation directions and are assembled into one-dimensional [VO₄]_∞ infinite chains by sharing an edge along the c-axis. Therefore, this is conducive to the production of large birefringence (as shown in Fig. 2b). Subsequently, the aforementioned 1D [VO₄]_∞ infinite chains are linked together by isolated [SeO₃] groups, resulting in the assembly of 2D [SeVO₅]_∞ infinite layers that extend along the bc-plane (shown in Fig. 2c). Finally, the 2D [SeVO₅]_∞ infinite layers were bound through Na–O ionic bonds and stacked along the a-axis (shown in Fig. 2d).

Fig. 2 (a) The [VO₆] octahedron and the trigonal pyramid [SeO₃] group. (b) 1D [VO₄]_∞ chain. (c) View of the 2D [VSeO₅]_∞ layer along the bc-plane. (d) The whole structure of NaVSeO₅ is viewed along the b-axis.

Spectroscopy analyses

UV-Vis-NIR diffuse reflectance spectroscopy showed that the crystal of NaVSeO₅ had almost no absorption in the range of 336 to 2500 nm.

The experimental band gap values of the crystals of NaVSeO₅ indicate almost no absorption in the range of 336 to 2500 nm. The experimental band gap values of the NaVSeO₅ were obtained at about 2.36 eV (shown in Fig. 3a). The IR spectra of compound NaVSeO₅ were measured in the 400–4000 cm⁻¹ wavenumber range at room temperature (shown in Fig. 3b). The infrared spectra of the compound exhibit both V–O and Se–O vibrations. The absorption band in the range of 764–930 cm⁻¹ can be classified as V–O bond vibration. The absorption band in the range of 526–620 cm⁻¹ as Se–O bond vibration is consistent with previously reported data.^66–68

Fig. 3 (a) UV-Vis-NIR diffuse reflectance and absorption spectra of NaVSeO₅; (b) IR spectrum of NaVSeO₅.

Thermal analyses

As shown in Fig. S1,† the thermogram indicates that the title compound has a continuous weight loss at intervals of 315 °C with slope changes. These weight losses correspond to the release of the SeO₂. Thermal behavior analysis and powder X-ray diffraction (XRD) data at various temperatures indicate that NaVSeO₅ remains thermally stable above 400 °C under open conditions (as shown in Fig. S2†).

Birefringence analysis

A high-quality crystal was intentionally selected to ensure more accurate birefringence measurements. The crystal reached extinction when the Berek compensator's drum was reversed, and the corresponding values were recorded. According to the Berek compensator's specification tables, the thickness of the selected crystals was 9.92 μm, and the optical path difference at 546 nm was 1.859 μm. The refractive index difference of NaVSeO₅ is 0.180@546 nm. The refractive index difference is known to be measured on the (100) crystal plane (shown in Fig. 4d). The experimental measurements range from zero to the birefringence value, as Δn represents the maximum refractive index difference. Comparison with other crystals in the same wavelength band reveals that the compound exhibits significant birefringence (shown in Fig. S3 and Table S5†).

Fig. 4 (a and b) Extinction views of the crystal by negative and positive rotational compensators; (c) view of the crystal thickness; (d) orientation of the NaVSeO₅ crystal determined by X-ray single-crystal diffraction.

Theoretical calculations and optical properties

To study the structure−property relationship of NaVSeO₅, first-principles calculations were used to analyze its electronic structures (shown in Fig. 5). It is clearly shown that NaVSeO₅ has an indirect band gap, and the theoretical band gap is about 1.87 eV, which is less than the value (2.36 eV) obtained from the transmittance spectrum (shown in Fig. 5a). The underestimation of the band gap drives from the inaccurately described unoccupied eigenvalues of electronic states.⁶⁹ Usually, the states near the band gap control the optical properties of a crystal, so the valence-band maximum (VBM) and the conduction band minimum (CBM) were carefully studied in the PDOS. The obtained PDOS is shown in Fig. 5b, the PDOS graph is divided into VB-I–II and CB energy regions. Se-4p and O-2p states dominate the VB-I region. O-2p and V 3d electronic states occupy VB-II. In CB, it is dominated by O-2p, V-3d, and Se-4p electronic states. Both VBM and CBM are mainly from the electronic states of V–O and Se–O. Additionally, we evaluated the birefringence of NaVSeO₅ through theoretical calculations. Fig. 5c indicates that the birefringence of NaVSeO₅ is about 0.189 at 546 nm. The birefringence values calculated for NaVSeO₅ are in good agreement with experimental data.

Fig. 5 (a) The birefringence of compound NaVSeO₅; (b) partial density of states (PDOS) of compound NaVSeO₅; (c) calculated band structure; (d) ELF of compound NaVSeO₅.

The RSAC method was used to further investigate the contributions of different groups to the refractive indices and birefringence. In the RSAC operation, the atomic radii were set according to the rule of keeping the cutting spheres in contact but without overlapping, and these values were set as 1.095 Å (Na), 0.98 Å (V), 1.82 Å (Se), and 1.10 Å (O). The obtained RSAC results are shown in Table 2, the birefringence of the Se–O polyhedral (0.221@546 nm) is larger than that of V–O (0.195@546 nm) polyhedra in NaVSeO₅, implying that Se–O polyhedra with asymmetric lone pair electrons give more contribution to total birefringence. The asymmetric lone pair electrons are further confirmed by the electron localization function (ELF). ELF intuitively reveals that there is a lobe-like isosurface around the Se⁴⁺ atom, which illustrates that lone pairs of Se⁴⁺ are stereochemically active (shown in Fig. 5d). Hence, it suggests that NaVSeO₅ has large anisotropic polarizabilities and birefringence.

Table 2 The functional basic units’ refractive indices and birefringence were obtained by RSAC methods

Compound		n x	n y	n z	Δn
NaVSeO5	Origin	2.269	2.229	2.080	0.189
	SeO3	2.054	1.834	1.833	0.221
	VO6	2.047	1.914	1.852	0.195

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Conclusions

In conclusion, a novel inorganic compound NaVSeO₅ with excellent birefringence properties has been successfully developed in a closed system using a synergistic strategy that involves multifunctional groups. In the structure of NaVSeO₅, the [VO₄]_∞ infinite chain is linked to isolated [SeO₃] groups, forming a regularly arranged two-dimensional [SeVO₅]_∞ infinite layer. Due to the strong SCALP effect of Se⁴⁺, the [VO₆] octahedra have a consistent deformation direction, giving the compound a significant birefringence (0.180 at 546 nm). UV-visible-near-infrared diffuse reflectance spectroscopy reveals that NaVSeO₅ has a broad transmission range with a cut-off edge below 336 nm. This study enriches the selenite family and offers insights for the exploration of novel birefringent crystals.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Major Science and Technology Projects of Xinjiang (Grant No. 2022A01005), the National Natural Science Foundation of China (12264047), and the Tianshan Talent Project of Xinjiang Uygur Autonomous Region of China (2022TSYCJU0004).

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Footnotes

† Electronic supplementary information (ESI) available: CIF files, atomic coordinates, equivalent isotropic displacement parameters and bond valence sum (BVS) and the table of bond lengths and angles for NaVSeO₅. CCDC 2334821. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00513a

‡ These authors contributed equally to this work.

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