“All-four-in-one”: a novel mercury tellurite–nitrate Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ exhibiting exceptional optical anisotropy

Abstract

In recent years, birefringent crystals have attracted much attention in the field of optical materials and play a significant role in laser technology and optical imaging. However, commercially available birefringent crystals are still relatively scarce and need improvement. Low-dimensional structures and well-oriented anisotropic units are conducive to obtaining excellent birefringent materials. Therefore, it is crucial to utilize a molecular engineering strategy for designing crystal structures. Through screening, the tellurite–nitrate system caught our attention because most of them exhibit low dimensional structures. The structure of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ (HTTN) consists of unprecedented [(Hg₃Te₄O₁₀)²⁺]_∞ cationic layers built by a [TeO₃]²⁻ triangular pyramid, [(Te₃O₇)²⁻]_∞ chains, and novel [(Hg₃O₇)⁸⁻]_∞ chains balanced by isolated NO₃⁻ anions. Here, HTTN with multiple functional units was obtained. HTTN has a birefringence value of 0.295 @ 546 nm, which is significantly higher than those of all commercially available birefringent crystals and exhibits the highest value among tellurite–nitrate birefringent crystals. Structural analysis and theoretical calculations reveal that the synergistic interaction between [TeO₃]²⁻ (5.19%) and [NO₃]⁻ (7.32%) groups and [(Te₃O₇)²⁻]_∞ (36%) and [(Hg₃O₇)⁸⁻]_∞ (51.49%) chains plays a crucial role in the optical anisotropy of HTTN. This study demonstrates that introducing functional units with high optical anisotropy is an effective strategy for developing high-performance birefringent materials.

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Introduction

Birefringent crystals have attracted widespread attention in the field of optical materials due to their ability to modulate light polarization. Since the first observation of birefringence, these functional crystals have become indispensable components in various linear optical devices and play a significant role in modern optoelectronic technology.¹ Currently, commercially available birefringent crystals include MgF₂,² α-BaB₂O₄,³ YVO₄,⁴ TiO₂,⁵ LiNbO₃ (ref. 6) and CaCO₃.⁷ However, these materials have some defects such as a small birefringence effect, susceptibility to phase transitions and cracking, difficulty in processing and a low laser damage threshold.^1g Therefore, there is an urgent need to develop new high-performance ones.

From the structural perspective, the large birefringence of most commercial birefringent crystals is primarily composed of π conjugated and highly distorted structural units. Besides, low-dimensional structures, particularly one-dimensional (0D) and two-dimensional (2D) structures, have a positive effect on enhancing optical anisotropy.^1h,j Therefore, it is crucial to utilize molecular engineering to design structures effectively. The presence of lone pair electrons allows cations to form highly distorted polyhedra, which not only exhibit significant anisotropic polarization but also effectively prevent connections with other groups on the same side, thus facilitating the formation of low-dimensional structures,⁸ such as Pb₂BO₃Cl, Sn₂PO₄I, and K₂Sb(P₂O₇)F.⁹ In addition, the presence of planar anionic groups is beneficial for the development of well-performing birefringent materials with low-dimensional structures, including π-conjugated groups such as (C₃N₆H₈)PbBr₄, Ba(H₂C₆N₇O₃)₂·8H₂O, LiRb(HC₃N₃O₃)·2H₂O, Be₂(BO₃)(IO₃), Sr₃Y[PO₄][CO₃]₃, Sn₂BO₃I, Sn₂B₅O₉Br, and LiZn(OH)CO₃.¹⁰ Obviously, inorganic π-conjugated BO₃ and CO₃ groups have positive impact on birefringence. NO₃ is very similar to BO₃ and CO₃, but there has been relatively less research on it despite its larger polarizability anisotropy.¹¹ Furthermore, the combination of lone pairs of nitrate ions helps form compounds with larger birefringence, such as Pb₂(BO₃)(NO₃)¹² and Bi₃TeO₆OH(NO₃)₂.¹³ In order to obtain compounds that can combine distorted lone-pair cation (Pb²⁺, Se⁴⁺, Sn²⁺, Te⁴⁺, etc.)-centered polyhedra and nitrate ions and exhibit low-dimensional structures, we screened the existing system (low-dimensional structures especially 1D or 2D and easy to combine with lone-pair cations). Through research, it has been discovered that the tellurite–nitrate system shows great potential in birefringent performance. Currently, there are only eleven tellurite–nitrates, among which low-dimensional structures account for as high as 80% (Table S5†). These results suggest that tellurite–nitrite salts have great potential in generating crystals with large birefringence characteristics. However, currently only (SbTeO₃)(NO₃) exhibits birefringence with a value of 0.078 at a wavelength of 546 nm,^14a indicating that there is still significant room for expansion in the study of birefringence for this system.

In recent years, many preeminent Hg-based birefringent crystals have been acquired, including BaHgSe₂, Hg₄(Te₂O₅)(SO₄), and HgBr₂.¹⁵ It is worth noting that Hg²⁺ ions can adopt various coordination geometries including linear, planar triangular or tetrahedra, providing diversity in structural variations. Furthermore, the significant distortion degree of Hg²⁺ ions contributes to the formation of large birefringence. Therefore, our main research focuses on introducing Hg²⁺ ions into the tellurite–nitrate system.

Based on the above thoughts, this study successfully prepared a novel mercury-based tellurite–nitrate Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ (HTTN) containing multiple functional units. It exhibits excellent birefringence. Combined with theoretical calculations, this paper presents the synthesis method, crystal structure and its relevant comparisons, optical properties, and structure–property relationships of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂.

Results and discussion

Using TeO₂, Hg(NO₃)₂·H₂O, ZnO, and HNO₃ as raw materials, transparent colorless HTTN single crystals were successfully synthesized through a hydrothermal reaction at 200 °C (ESI† for more details). The powder X-ray diffraction (PXRD) characterization result shown in Fig. S1† demonstrates good agreement with the simulated one, confirming the purity. The chemical composition was further supported by EDS elemental analysis presented in Fig. S2.†

Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ belongs to the orthorhombic crystal system with the Pnma space group (No. 62). The asymmetric unit consists of three Hg, three Te, two N, and twelve O atoms (Fig. S6a†). In the crystal structure, each N atom is coordinated with three O atoms to form a NO₃ planar triangular geometry (Fig. S6a†). The bond distances of N–O are from 1.18(2) to 1.28(2) Å, and the O–N–O bond angles are between 118.4(18) and 122.3(17)°. Hg(1) forms a HgO₅ triangular prism, whereas Hg(2) forms a HgO₄ tetrahedron. Two HgO₄ and one HgO₅ units are combined through the O(3) and O(5) atoms to form a [Hg₃O₉] trimer. Subsequently, the [Hg₃O₉] units connect with each other along the b-axis through sharing O atoms to form an S-shaped [(Hg₃O₇)⁸⁻]_∞ chain (Fig. 1a). The chains are arranged consistently on the ab plane and stacked along the c-axis (Fig. S7a†). Specifically, this is the first discovery of the [Hg₃O₉] trimer and [(Hg₃O₇)⁸⁻]_∞ chain. The distances of Hg–O bonds range from 2.11(8) to 2.578(8) Å. The distances of Hg–O bonds range from 2.11(8) to 2.578(8) Å, and the angle of O–Hg–O ranges from 63.2(3) to 170.1(4)°. Both Te(1) and Te(2) atoms are connected to four O atoms, forming TeO₄ tetrahedra. The Te(3) atom is connected to three O atoms, forming a TeO₃ triangular pyramid. It is arranged in an inverted parallel manner in the ab plane and extends infinitely along the c axis (Fig. 1b and S7b†). The distances of Te–O bonds range from 1.877(8) to 2.349(2) Å, and the O–Te–O bond angles range from 82.7(8) to 153.9(4)°. All bond lengths and angles observed in this compound are very similar to those found in other compounds. The TeO₃ units, [(Te₃O₇)²⁻]_∞ chains, and [(Hg₃O₇)⁸⁻]_∞ chains are connected together to form wrinkled [(Hg₃Te₄O₁₀)²⁺]_∞ layers in the bc plane, with NO₃⁻ ions between the layers to balance the charge (Fig. 1d and e). Interestingly, the [Te₃O₇]²⁻ chain, [Hg₃O₇]⁸⁻ chain, and [TeO₃]²⁻ unit form a collective arrangement that remains nearly coplanar (Fig. 1d). In the [(Hg₃O₇)⁸⁻]_∞ chain, Hg atoms are in the same plane (Fig. 1a), while Te⁴⁺ cations in both the [TeO₃]²⁻ and [TeO₄]⁴⁻ units are roughly aligned along the b-axis (Fig. S7b†). Meanwhile, the NO₃ ions maintain a coplanar arrangement along the ac or bc plane with different orientation modes (Fig. S6b†). The arrangements of these structural units are conducive to strong optical anisotropy.

Fig. 1 (a) The [(Hg₃O₇)⁸⁻]_∞ chain; (b) the TeO₃ units; (c) the [(Te₃O₇)²⁻]_∞ chain; (d) the [(Hg₃Te₄O₁₀)²⁺]_∞ layer; and (e) the whole crystal structure of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂.

So far, only eleven tellurite–nitrates have been reported (Table S5†). Furthermore, only (SbTeO₃)(NO₃) has been studied with birefringent performance,^14a with a value less than 0.1. For tellurite–nitrates with 2D structures, in (SbTeO₃)(NO₃),^14a AgTeO₂NO₃,^14b [Bi(TeO₃)](NO₃),^16a (Te₂O₄)(HNO₃),^14e and YCu(TeO₃)₂(NO₃)(H₂O)₃,^14d the NO₃ ions mainly exist between layers. For 2D RE(TeO₃)(NO₃) (RE = La, Nd, Eu, Gd, Dy, Er and Y),^16b 0D Ca₅Te₄O₁₂(NO₃)₂(H₂O)₂ and 3D Ca₆Te₅O₁₅(NO₃)₂,^16c the NO₃⁻ groups are connected to the cations RE³⁺ and Ca²⁺, respectively. In 0D Bi₃(μ₃-OH)(TeO₃)₃(NO₃)₂ (ref. 16d) and H[Bi₃O(Te₃O₉)](NO₃)₂,^14c the NO₃ units are free between chains. It can be seen that different positions of NO₃⁻ lead to significant changes in structural dimensions which may affect birefringence. In the above compounds, Te atoms exhibit different coordination forms: TeO₂, TeO₃, and TeO₄ units. Additionally, there are Te₂O₂ rings and [(Te₃O₁₀)⁸⁻]_∞, (TeO₂)_∞ and [(TeO₄)⁴⁻]_∞ chains. Obviously, Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ includes both TeO₃ and TeO₄ units along with [(Te₃O₇)²⁻]_∞ chain in the tellurate–nitrate system for the first time. Simultaneously, the existence of Hg₃O₉ units and [(Hg₃O₇)⁸⁻]_∞ chains is also observed for the first time.

Fig. S3† shows the TG curves of HTTN, which suggest that HTTN can be stable below 373 °C, more stable than Bi₃(μ₃-OH)(TeO₃)₃(NO₃)₂ (180 °C),^16d (SbTeO₃)(NO₃) (273 °C),^14a and [Bi(TeO₃)](NO₃) (370 °C).^16a

The optical band gap of HTTN was determined to be 3.5 eV based on UV-vis-NIR diffuse reflectance spectroscopy and the Kubelka–Munk function (Fig. 2). The band gap value is higher than that of Bi₃(μ₃-OH)(TeO₃)₃(NO₃)₂ (3.31 eV) and H[Bi₃O(Te₃O₉)](NO₃)₂ (3.26 eV).^16d,14c The infrared spectrum and vibration peak distribution of HTTN are shown in Fig. S5.† There are no obvious vibration peaks in the range of 1500–4000 cm⁻¹. The strong bands at 1401 and 1295 cm⁻¹ correspond to the N–O stretching vibrations of the triangular NO₃ group, while those at 1039 and 817 cm⁻¹ correspond to non-planar bending vibrations of the NO₃ plane group. The peaks at 759 and 696 cm⁻¹ can be attributed to Te–O stretching, while those at 609 and 518 cm⁻¹ can be attributed to symmetric and asymmetric stretching of Hg–O bonds. These vibration peaks are close to those in the literature.¹⁷

Fig. 2 UV-vis-NIR diffuse reflectance spectra and the optical band gaps obtained from Tauc plots (inset) of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂.

In order to further investigate the potential of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ as a birefringent material, its birefringence was measured using a polarizing microscope equipped with a 546 nm light source. The thickness of the crystal was 3.95 μm, and the optical path difference at a wavelength of 546 nm was determined to be 1.16684 μm (Fig. 3b and c).^14a According to the formula “R = Δn × T”, the birefringence can be calculated. The result showed that the birefringence of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ at 546 nm is 0.295 (Fig. 3a). A statistical analysis on all inorganic nitrates with experimentally determined birefringence was performed, and the results show that Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ shows higher birefringence compared with Pb₆O₄(BO₃)(NO₃) (0.276 @ 546 nm), In(IO₃)₂(NO₃) (0.268 @ 546 nm), [Al(H₂O)₆](IO₃)₂(NO₃) (0.252 @ 546 nm), Pb₆O₂(BO₃)₂(NO₃)F (0.241 @ 546 nm), etc.¹⁸ Additionally, the birefringence of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ exceeds that of commercially available α-BaB₂O₄ (0.122 @ 546 nm),³ CaCO₃ (0.172 @ 546 nm),⁷ YVO₄ (0.204 @ 532 nm),⁴ TiO₂ (0.256 @ 546 nm),⁵ and some other common birefringent crystals (Table S4†). Furthermore, its birefringence value surpasses that of all known birefringence tellurite–nitrates, being 3.78 times greater than (SbTeO₃)(NO₃) (0.078 @ 546 nm) (Fig. 4 and S8†).^14a In conclusion, Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ shows great potential as a birefringent material.

Fig. 3 (a) Experimental birefringence at 546 nm. (b) Polarization optical microscope image of a Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ single crystal. (c) A single crystal corresponding to complete extinction. (d) Calculated refractive index dispersion curves of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂.

Fig. 4 Birefringence of tellurite–nitrate crystals.

In order to accurately elucidate the relationship between the structure and performance, first-principles calculations were conducted. The results show that Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ exhibits a direct band gap of 2.4 eV (Fig. S4a†). Due to the limitation of the GGA-PBE method, the calculated band gap value is underestimated compared to experimental values.^1i,j Therefore, we used a scissor operator of 1.1 eV to evaluate the optical properties of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂. The density of states distribution for Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ is shown in Fig. S4b,† where the top region of the valence band is mainly occupied by O-2p and Hg-5d orbitals and some Te-5s and Te-5p orbitals. The bottom region of the conduction band is primarily filled with O-2p, Te-5p, Hg-6s and some N-2p orbitals. From this it can be inferred that charge transfer between valence and conduction bands is mainly determined by Hg, Te, and O atoms. It can be found from the valence band to the conduction band that there is a strong overlap between the electronic states of Te and Hg atoms with those of O atoms, indicating a strong interaction between Te–O and Hg–O bonds. To further confirm the origin of the birefringence, an atom-resolved birefringence analysis was conducted. The birefringence in Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ is attributed to [TeO₃]²⁻ (5.19%), [NO₃]⁻ (7.32%) groups, [Te₃O₇]²⁻ (36%) , and [Hg₃O₇]⁸⁻ (51.49%) chains. This further confirms that the synergistic strategy involving four functional groups results in the compound possessing significant birefringence (Fig. 5).¹⁹

Fig. 5 Calculated birefringence contribution from [TeO₃]²⁻, [NO₃]⁻ groups, [Te₃O₇]²⁻, and [Hg₃O₇]⁸⁻ chains.

Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ crystallizes in the orthorhombic crystal system and belongs to the biaxial crystal system, with three refractive indices (n_x, n_y, and n_z) along the x, y, and z axes respectively. The refractive indices of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ satisfy the relationship n_y > n_x > n_z. According to theoretical calculations, the birefringence value Δn at a wavelength of 546 nm is determined to be 0.27, which is close to the experimental result (Fig. 3d).

In order to gain a deeper understanding of the contributions of different functional groups to the optical anisotropy of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂, we calculated the electron localization function maps for Hg₃(TeO₃)(Te₃O₇)(NO₃)₂. According to Fig. 6, obvious distortion can be observed in the electron clouds surrounding the [TeO₃]²⁻ and [NO₃]⁻ groups, as well as the [(Te₃O₇)²⁻]_∞ and [(Hg₃O₇)⁸⁻]_∞ chains. According to the research by Pan's team, the contribution of lone pairs to birefringence becomes more significant as cos θ (the angle between the direction of lone pairs and the optical axis n_min) increases.²⁰Fig. 6b shows that when lone pair electrons are parallel to the optical axis n (determined by selecting the refractive index with the smallest value among the three axes calculated using DFT), the cos θ factor is 1. Therefore, observing TeO₃ and Te₃O₇ units with directions of lone pair electrons in this compound helps observe larger birefringence. Therefore, it can be concluded from further analysis of Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ difference charge density maps that prominent optical anisotropy mainly arises from a synergistic interaction among the [TeO₃]²⁻ and [NO₃]⁻ groups, as well as the [(Te₃O₇)²⁻]_∞ and [(Hg₃O₇)⁸⁻]_∞ chains.

Fig. 6 (a) The electron localization function diagram. (b) Arrangement of the [TeO₃]²⁻ unit and [Te₃O₇]²⁻ chains in Hg₃(TeO₃)(Te₃O₇)(NO₃)₂. Blue crescents indicate the approximate position of lone-pair electrons. The downward and rightward arrows respectively represent the minimum and maximum refractive indices along the c-axis and b-axis directions.

Conclusions

In conclusion, we successfully synthesized a novel mercury-based tellurite–nitrate Hg₃(TeO₃)(Te₃O₇)(NO₃)₂. Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ is the first tellurite–nitrate compound generated by multiple functional units that exhibits excellent optical anisotropy. It possesses a unique crystal structure composed of [TeO₃] triangular pyramids, [NO₃]⁻ units, [(Te₃O₇)²⁻]_∞ chains, and [(Hg₃O₇)⁸⁻]_∞ chains. The research results indicate that Hg₃(TeO₃)(Te₃O₇)(NO₃)₂ possesses a wide bandgap (3.5 eV) and an excellent birefringence (0.295 @ 546 nm). This represents the largest birefringence in tellurite–nitrate, surpassing that of commercially available birefringent crystals. Additionally, theoretical calculations and structural analysis indicate that the synergistic interactions between [TeO₃]²⁻ and [NO₃]⁻ groups, as well as between [(Te₃O₇)²⁻]_∞ and [(Hg₃O₇)⁸⁻]_∞ chains, are the main factors causing the optical anisotropy. This work significantly enriches the family of nitrate compounds and has important implications for further understanding and exploring nitrate crystal structures and developing high-quality birefringent materials. It provides new insights for future design and research on excellent birefringent materials.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

This work was conceptualised by Ru-Ling Tang. Experimentation was performed by Yi-Lei Lv and Ru-Ling Tang. Software analysis was performed by Liang Ma. Bing-Wei Miao has experimental assisted contribution. Wenlong Liu contributed supervision. Besides, Ru-Ling Tang and Sheng-Ping Guo contributed funding acquisition and supervision. The first draft of the manuscript was prepared by Ru-Ling Tang and Yi-Lei Lv, and the final draft was edited by all the authors.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (22101248), the Lvyangjinfeng Talent Program of Yangzhou (YZLYJFJH2021YXBS083), and the Qinglan Project of Yangzhou University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The experimental section and additional tables and figures. CCDC 2400692. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc08166h

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