HgBr₂: an easily growing wide-spectrum birefringent crystal

Abstract

Birefringent materials are of great significance to the development of modern optical technology; however, research on halide birefringent crystals with a wide transparent range remains limited. In this work, mercuric bromide (HgBr₂) has been investigated for the first time as a promising birefringent material with a wide transparent window spanning from ultraviolet (UV) to far-infrared (far-IR) spectral regions (0.34–22.9 μm). HgBr₂ has an exceptionally large birefringence (Δn, 0.235 @ 546 nm), which is 19.6 times that of commercial MgF₂. The ordered linear motif [Br–Hg–Br] with high polarizability anisotropy within the molecule is the inherent source of excellent birefringence, making it an efficient building block for birefringent materials. In addition, HgBr₂ can be easily grown under mild conditions and remain stable in air for prolonged periods. Studying the birefringent properties of HgBr₂ crystals would provide new ideas for future exploration of wide-spectrum birefringent materials.

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Introduction

Birefringence is manifested by the existence of orientation-dependent differences in the refractive index, which is widely used in the fields of optical polarization, optical modulation, and nonlinear optics.^1–7 The exploration of birefringent materials is an essential topic in the field of optical research.^8–15 To date, a diverse range of oxide and halide birefringent materials with high chemical stability have garnered significant interest, including MgF₂,¹⁶ α-BaB₂O₄,¹⁷ YVO₄,¹⁸ TiO₂,¹⁹ LiNbO₃,²⁰ CaCO₃,²¹etc. However, the aforementioned birefringent materials are plagued by various inherent defects: MgF₂ possesses a tiny birefringence; α-BaB₂O₄ encounters issues such as phase transition and cracking during growth; TiO₂ demonstrates high hardness and poses challenges in terms of processing; YVO₄ cannot be utilized within the ultraviolet band; LiNbO₃ exhibits a low laser-induced damage threshold and is susceptible to photorefractive damage; CaCO₃ is deliquescent and difficult to obtain high-quality single crystals. Hence, it holds immense significance for practical applications to discover birefringent materials with substantial birefringence, multi-wave applicability, excellent chemical and physical stability, as well as favorable crystal growth habits.

Metal halides are highly regarded as important optical function materials due to their advantages of easy preparation, rich coordination environment, wide transparent range, high laser-induced damage threshold, and are applied in the frontier fields of luminescence, solar cells, laser frequency conversion, and so on.^22–29 Among them, binary metal halides are widely utilized owing to their simple composition and cost-effectiveness: KBr is commonly employed as a background material in Fourier transform infrared (FT-IR) spectroscopy due to its wide transparent range exceeding 25 μm;³⁰ CaF₂ and BaF₂ exhibit excellent mechanical properties, thermal stability, and radiation resistance along with high transparency from deep ultraviolet (UV) to IR regions, which are applied in optical prisms, lenses, wedge plates, diaphragms and other important optical components.³¹ By reason of the foregoing, the splendid physical and chemical properties of binary metal halides are in line with our expectations for the next generation of birefringent crystal materials, making them regarded as a treasury of birefringent materials with great potential. On the other hand, metal halides display diverse coordination patterns, including linear, trigonal pyramidal, tetrahedral, and square pyramidal structures, which offer promising opportunities for identifying building blocks with significant polarizability anisotropy for birefringent materials.²⁵ Through comparison and screening, binary mercury-based (Hg-based) halides have emerged as a focal point for us due to their abundant configurations and broad transparent range. In Hg-based halides, apart from the traditional [HgX₄] (X = halogen) tetrahedra, there also exist infrequent [X–Hg–X] or [X–Hg–Hg–X] linear units.^32–36 In borates, the linear unit [BO₂] is thought to have greater polarizability anisotropy than the classic [BO₃],^37,38 and the previous studies of HgB₂S₄ and trigonal HgS have suggested that the [S–Hg–S] linear units have strong birefringent contributions (0.28 at 1064 nm and 0.29 at 2100 nm, respectively).^39,40 Moreover, the [X–Hg–Hg–X] motif in the binary mercurous halide can induce a giant birefringence; however, prolonged exposure to light can lead to decomposition of mercurous halide.⁴¹ Therefore, the linear [X–Hg–X] unit can also be regarded as a potential emerging building block of birefringent materials; however, its comprehensive investigation remains insufficient. Moreover, binary mercuric halides offer greater advantages in terms of synthesis and crystal growth, with their growth conditions generally being more moderate, thereby facilitating the achievement of large-scale crystal growth and device fabrication. Consequently, simple halides composed of linear [X–Hg–X] units are promising birefringent materials. In this work, mercuric bromide was selected as a potential birefringent material for the first time. A colorless single-crystal of mercuric bromide with a size of 4 × 3 × 2 mm³ was successfully obtained through a slow solvent evaporation technique employing hot ethanol as the solvent (as shown in Fig. 1a). The birefringence of HgBr₂ at 546 nm is 0.235, which is 19.6 times higher than that of another commercial binary halide material, MgF₂ (0.012 @ 546 nm).¹⁶ Moreover, HgBr₂ demonstrates a transparent range spanning from 0.34 to 25 μm, effectively covering an ultrawide transparent range from UV to far-IR. The rationality of the experimental results is confirmed by theoretical calculations, indicating the potential of mercuric bromide as a next-generation birefringent material.

Fig. 1 (a) The single-crystal HgBr₂ with a size of 4 × 3 × 2 mm³; (b) the structure of HgBr₂ viewed along the a-axis (left) and the coordination environment of the Hg atom (right).

Results and discussion

We have computationally determined the polarizability anisotropy and HOMO–LUMO gap of five Hg-based linear units (three [HgX₂], X = Cl, Br, I; two [HgQ₂], Q = S, Se), aiming to substantiate their potential as birefringent materials and analyze their respective merits and demerits. As depicted in Fig. 2a, the polarizability anisotropy of most Hg-based linear units is comparable to or even surpasses that of [BS₃], the novel IR birefringent material building unit. Among them, [HgX₂] exhibits a wider HOMO–LUMO gap compared to [BS₃], which is advantageous for achieving a wider band gap and a higher laser-induced damage threshold for meeting the application requirements of materials in high-energy laser systems. For instance, previous studies have demonstrated that the laser-induced damage threshold of HgBr₂ reaches 0.3 GW cm⁻²,³⁵ significantly surpassing those of the [BS₃]-based compounds, such as Ca₂La(BS₃)(SiS₄) (34.43 MW cm⁻²), LaBS₃ (49.55 MW cm⁻²) and BaB₂S₄ (265 MW cm⁻²).^42–44 Although the polarization anisotropy of [HgQ₂] is 1.7 to 2 times that of [BS₃], their HOMO–LUMO gap is narrower, which would affect their optical band gap. For the above-mentioned reason, the [HgX₂] units are potential building units with comparable performance to [BS₃], suggesting that Hg-based binary halides composed of [HgX₂] may serve as possible wide-spectrum birefringent materials. Considering the hydrolysis susceptibility of HgCl₂ and the narrow band gap exhibited by HgI₂, we selected HgBr₂ as our subject for investigating birefringence properties.

Fig. 2 (a) Polarizability anisotropy and HOMO–LUMO gaps of the [BS₃], [HgCl₂], [HgBr₂], [HgI₂], [HgS₂], and [HgSe₂] units; (b) experimental Raman spectrum of HgBr₂; (c) UV-vis-IR transmittance spectrum collected from the single crystal (inside: the photograph of single crystal HgBr₂ plate); (d) crystallographic orientation and birefringence measurement of the HgBr₂ crystal; (e) the calculated birefringence values of HgBr₂.

The crystal growth procedure of HgBr₂ is as follows: a saturated solution of mercury bromide was prepared in a beaker using hot anhydrous ethanol as the solvent under continuous agitation. Subsequently, the clear solution was filtered while still hot into another beaker, and the solvent was very slowly evaporated at room temperature. After one month, a substantial quantity of crystals with two to four millimeter-sized were successfully acquired. The analytical results based on the crystallographic data obtained by single-crystal XRD demonstrate that HgBr₂ crystal crystallizes in the orthorhombic space group Cmc2₁, and the cell parameters are: a = 4.6215(6) Å, b = 6.7794(7) Å, c = 12.4277(19) Å, and Z = 4, which are consistent with the results reported in previous literature.⁴³ The structure of HgBr₂ is a simple zero-dimensional (0D) arrangement consisting of isolated [HgBr₂] units (Fig. 1b), with one crystallographically independent Hg atom and two Br atoms in each asymmetric unit. The bond lengths of Hg–Br range from 2.427(6) Å to 2.445(7) Å, and the angle of ∠Br–Hg–Br is 179.58°, both of which align with the corresponding values documented in the literature.⁴⁵ In general, the majority of three-dimensional (3D) structural frameworks constructed from metal-centered coordination tetrahedra typically exhibit low anisotropy and small birefringence, whereas lower-dimensional structures tend to display significant anisotropy and consequently induce appreciable birefringence.⁴⁶ Therefore, the combination of the 0D structure of HgBr₂ and the 1D linear building unit [HgBr₂] with a large polarizability anisotropy will facilitate the generation of large birefringence.

The optical band gap of HgBr₂ gained by UV-vis-NIR diffuse-reflectance spectroscopy processed by the Tauc plot method was 3.60 eV (Fig. 2c),⁴⁷ which was slightly higher than the band gap reported in previous literature (3.3 eV),³⁵ larger than those of some commercial IR optical materials such as AgGaS₂ (2.76 eV), AgGaSe₂ (1.83 eV),⁴⁸ ZnSe (2.58 eV) and comparable to that of ZnS (3.72 eV).^49,50 The wider optical band gap can effectively mitigate the damage caused by photon absorption to the material, thereby enhancing the laser-induced damage threshold for application in high-energy laser systems. In addition, according to the diffuse reflection spectrum, the UV cutoff edge of HgBr₂ was about 344 nm. Combined with the IR spectrum, it can be determined that the transparent range of HgBr₂ was 0.34–22.9 μm (Fig. 2c), covering a broad spectral range from UV to far-IR, surpassing those of all commercially available birefringent materials. The wide transparency range of HgBr₂ can be attributed to two aspects: the substantial atomic weight of mercury and bromine atoms leads to their low phonon energy thus enabling high transmittance in the far-IR region; additionally, the high electronegativity of bromine leads to a strong bond of valence electrons, consequently yielding a broad optical band gap. In addition, the sharp absorption peak at 4.2 μm in Fig. 2c originates from the asymmetric stretching vibration of the C=O double bond in carbon dioxide molecules; the dense and sharp absorption peaks observed within the range of 5–8 μm may be attributed to the bending vibrations of hydroxyl groups in gaseous water molecules; the two minor absorption peaks detected at wavelengths of 9.5 μm and 11.3 μm can be ascribed to stretching and bending vibrations associated with C–O single bonds present in ethanol molecules. Given our utilization of ethanol as a solvent for crystal growth purposes, it is possible for weak IR absorption features related to ethanol to emerge within the IR spectrum. The Raman spectrum of HgBr₂ is shown in Fig. 2b. The peak at 187 cm⁻¹ has roots in the vibration of the Hg–Br bond,⁵¹ and no other peaks are found in the range from 4000 to 100 cm⁻¹.

To thoroughly investigate the potential of HgBr₂ as a birefringent material, crystal HgBr₂ was measured using a polarizing microscope equipped with a 546 nm light source. The crystal of HgBr₂ chosen for the birefringence test is depicted in Fig. 2d. The crystallographic plane of the single crystal for birefringence measurement was confirmed as (001) through orientation using single-crystal XRD. Referring to the Michel-Levy Birefringence Chart, the optical path difference of the measured crystal was 9411.707 nm, and its thickness was 0.04 mm, and thus a calculated value of 0.235 can be assigned to the refractive index difference of this crystal under 546 nm illumination, which surpassed that of commercially available birefringent material MgF₂ (0.012)¹⁶ and even those of recently reported binary halides SbCl₃ (0.172) and α-SnF₂ (0.177) containing stereochemical activity lone pairs.⁵² Since HgBr₂ crystallized in the orthorhombic system and belonged to a biaxial crystal, there were three refractive indices (n_x, n_y, and n_z) along the x-, y-, and z-axes, respectively. The relationship between these refractive indices was given as n_z > n_y > n_x. The maximum birefringence of HgBr₂ occurred at the (010) crystallographic plane, which corresponded to the difference between n_z and n_x. However, due to the intrinsic growth habit of HgBr₂, it was challenging to expose the (010) plane effectively. Despite attempts to mechanically cut the crystal to obtain an exposed (010) plane, we have been unsuccessful in producing a wafer that met the testing conditions of a polarizing microscope. Therefore, considering that the measured crystal plane of HgBr₂ was not parallel to the optical axis plane, the birefringence obtained at the (001) crystal plane should be less than or equal to n_z − n_x and close to n_y − n_x. As presented in Fig. 2e, the maximum calculated refractive index difference (n_z − n_x) of HgBr₂ was 0.245 at 546 nm and the value of n_y − n_x was 0.195, which exhibited good agreement with the experimental birefringence measurement and our conjecture. Despite challenges associated with exposing specific crystallographic planes in the HgBr₂ crystal for accurate measurement purposes, our findings demonstrate that there is still agreement between theoretical calculations and experimental results when it comes to understanding its birefringent properties on other crystal planes, which has not received enough attention in previous similar studies.

We have conducted a statistical analysis for classical birefringent materials (MgF₂, α-BaB₂O₄, YVO₄, TiO₂, LiNbO₃, CaCO₃),^16–21 recently reported birefringent materials with large crystals (K₂SbMoO₂F₇, NaGaS₂),^46,53 common commercial nonlinear optical materials (LiB₃O₅, β-BaB₂O₄, KH₂PO₄, KTiOPO₄, AgGaS₂, AgGaSe₂, ZnGeP₂, CdGeAs₂),⁴⁸ and some chalcogenide optical materials (ZnS, CdS, CdSe)^54–56 to compare them with HgBr₂ in terms of transparent range and birefringence. Note that the transparent spectra of these materials were tested based on large crystals, ensuring high reliability. Additionally, most of the refractive index tests for these materials were performed using an ∼550 nm light source, except for narrow band gap materials including AgGaSe₂ (@2000 nm), CdSe (@2000 nm), ZnGeP₂ (@2000 nm) and CdGeAs₂ (@3000 nm). From Fig. 3a, it is evident that HgBr₂ ranked among the top in both the spectral transparent range and birefringence among these optical materials; specifically, HgBr₂ possessed a transparent range that exceeds those of most birefringent materials, and its birefringent performance is second only to TiO₂ among commercial birefringent materials, which demonstrated that HgBr₂ has great potential as a wide-spectrum birefringent material.

Fig. 3 (a) Comparison of HgBr₂ with typical optical crystals in terms of birefringence and transmission range; (b) the calculated band structure (left) and density of states (DOS, right) of HgBr₂; (c) 2D ELF diagram through the [Br–Hg–Br] motif for HgBr₂.

In order to better understand the relationship between the structure and properties, the electronic structure of HgBr₂ has been calculated based on the first-principles method. As depicted in Fig. 3b (left), HgBr₂ was a direct bandgap semiconductor with a calculated band gap of 2.57 eV, which slightly deviates from the experimental optical band gap (3.60 eV) due to the inherent limitation of the GGA-PBE functional caused by the discontinuity of exchange correlation energy, resulting in an underestimation of the band gap.⁵⁷ However, this value is consistent with the previous calculation by Kanchana et al. without using the TB-mBJ functional (2.24 eV).⁵⁸ The DOS of HgBr₂ is shown in Fig. 3b (right). The top of the valence band (VB) and the bottom of the conduction band (CB) were mainly governed by Hg-6s, Hg-5d, and Br-4p orbitals, indicating that the optical properties of HgBr₂ were predominantly influenced by [HgBr₂] units. The electron localization function (ELF) maps projected on crystal planes parallel to [HgBr₂] motifs were further subjected to analysis. As illustrated in Fig. 3c, the electron cloud around the [HgBr₂] motif exhibited a dumbbell-like shape and demonstrates large optical anisotropy, which was the primary cause of the significant birefringence in HgBr₂. The above results further validated the rationality of the experimental findings.

Conclusions

In summary, we have investigated HgBr₂ as a wide-spectrum birefringent material for the first time and successfully obtained a large number of millimeter-scale single crystals through simple and mild growth methods. The findings revealed that HgBr₂ possessed a broad band gap (3.60 eV) and transparent range (0.34–22.9 μm), and notably, it displayed a large birefringence that is 19.6 times that of the commercial halide MgF₂, thereby demonstrating HgBr₂ as a promising wide-spectrum birefringent material. Furthermore, we substantiated for the first time via theoretical calculations that the Hg-based linear unit possessed a significant polarizability anisotropy, qualifying it as a potential candidate for constructing superior birefringent materials. This work will provide a novel idea for the design and exploration of wide-spectrum birefringent materials in the future.

Data availability

Data available on request from the authors.

Author contributions

Ming-Shu Zhang: conceptualization, data curation, visualization, writing – original draft; Wen-Dong Yao: theoretical calculations; Shao-Min Pei: data curation, theoretical calculations; Bin-Wen Liu: writing – review & editing; Xiao-Ming Jiang: software; Guo-Cong Guo: supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21827813, 21921001, 22175172, 22075283, 92161125, and U21A20508), the Youth Innovation Promotion Association of Chinese Academy of Sciences (2020303 and 2021300), and Fujian Science & Technology Innovation Laboratory for Optoelectronic Information of China (grant no. 2020ZZ108).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, crystallographic data, EDS and PXRD patterns of HgBr₂. See DOI: https://doi.org/10.1039/d4sc00836g

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