Growth and Temperature-Dependent Mechanical and Thermal Properties of One-Dimensional Chain Structure Hg₂Br₂ Crystals for Infrared Acousto-Optic Device Application

Abstract

The long-wave infrared acousto-optic tunable filters (AOTFs) based on Hg2Br2 crystals have attracted much interest in space exploration and harmful chemical agent detection. However, the growth of large Hg2Br2 single crystals with high quality and optical homogeneity still remains a challenge, and the lack of temperature-dependent mechanical and thermal properties of this crystal hinders the further enhancement of acousto-optic performance in practical applications. In this context, Hg2Br2 crystals (Φ 35 × 40 mm3) were successfully grown by the physical vapor transport (PVT) method. The optical homogeneity of the as-grown crystal is increased to 7.15 × 10–5 by restraining the impurities, striations, and cracks. In addition, the anisotropic mechanical and thermal properties varying with the temperature of Hg2Br2 crystals were systematically investigated. The compression modulus along the ⟨110⟩ orientation decreases from 17 to 7 GPa in the temperature range of −100 to 150 °C, while it ranges from 5.5 to 1.5 GPa along the ⟨001⟩ orientation. The thermal conductivities along ⟨110⟩ and ⟨001⟩ orientations vary in the range of 1.32–0.52 and 2.06–1.05 W/(m·K) with the increase in temperature from −140 to 100 °C, respectively, and then remain stable in the range of 100–150 °C. Using the first-principles calculation on the electron transfer characteristic between atoms and the bonding property within molecules of Hg2Br2 crystals, the large anisotropic mechanical and thermal properties could be explained, which are mainly attributed to the one-dimensional chain structure of the compound. Notably, the mechanical and thermal properties of Hg2Br2 crystals at low temperature are better than those at high temperature, which guarantees the practical application of AOTFs in a space extreme environment.

Single crystal growth and effect of cleavage micro-striations on the crystallinity and optical properties of mercurous halide single crystals

“…Though the Hg 2 I 2 crystals have a large acousto-optic figure of merit (3200 × 1.5 × 10 −18 s 3 g −1 ) and a wide spectral transmission range (0.5-40 μm), large-size Hg 2 I 2 crystals with high-quality are extremely difficult to obtain, and these crystals are prone to cleave because of their severe layered characteristics. 4 Therefore, Hg 2 Cl 2 and Hg 2 Br 2 crystals were chosen as the research objects in this work.…”

“…Compared with the polarizing and acousto-optic materials above, one dimensional chain-like Hg 2 X 2 (X = Cl, Br, and I) crystals are a class of potential polarizing and acousto-optic materials with excellent performance in the long-wave infrared range, for their ultra-large birefringence (0.547@10.6 μm for Hg 2 Cl 2 crystals and 0.667@10.6 μm for Hg 2 Br 2 crystals) and wide transparent ranges (0.38-20 μm for Hg 2 Cl 2 and 0.42-30 μm for Hg 2 Br 2 ). 4 Some studies on crystal growth, property characterization and device fabrication of Hg 2 X 2 crystals have been reported. [5][6][7][8][9][10][11] In the early 1970s, Barta et al reported the growth, the polarizing prism design, the refractive index and the velocity of Hg 2 Cl 2 and Hg 2 Br 2 crystals.…”

“…1,[17][18][19][20] In 2022, our group reported the photochromic phenomenon, the temperaturedependent anisotropic mechanism, and the thermal properties of Hg 2 Br 2 crystals. 4,21 In addition, first-principles calculations were performed to explore the origin of ultrabroad IR transmission and ultra-large birefringence in the Hg 2 X 2 family. 22 However, the achievable size and manufacturing of Hg 2 X 2 crystals currently still hindered the practical application of AO devices.…”

“…25 The smallest full width at half maximum (FWHM) of the polished Hg 2 Br 2 crystal wafer along the 〈110〉 orientation reported by our group is 0.29°, revealing the relatively low crystalline quality. 4 Owing to the fact that crystal processing is closely related to the performance of AO and polarizing devices in practical applications, 14,[24][25][26][27][28] it is necessary to explore the large size single crystal growth and the optimal processing method for Hg 2 X 2 crystals.…”

“…Compared with the polarizing and acousto-optic materials above, one dimensional chain-like Hg 2 X 2 (X = Cl, Br, and I) crystals are a class of potential polarizing and acousto-optic materials with excellent performance in the long-wave infrared range, for their ultra-large birefringence (0.547@10.6 μm for Hg 2 Cl 2 crystals and 0.667@10.6 μm for Hg 2 Br 2 crystals) and wide transparent ranges (0.38–20 μm for Hg 2 Cl 2 and 0.42–30 μm for Hg 2 Br 2 ). 4…”