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.