Temperature dispersions of refractive indices and absorption coefficients of mercury thiogallate crystals in the terahertz frequency range

THz absorption and refraction spectra of HgGa$_2$S$_4$ crystals of various compositions in the temperature range of 300–400 K have been studied, and a change in refractive indices for crystals depending on their stoichiometry has been found. For non-stoichiometric and stoichiometric samples, $\Delta n/\Delta T$ varies more than twice from 1.3 $\cdot$ 10$^{-4}$ to 3.1 $\cdot$ 10$^{-4}$ K$^{-1}$. The absorption coefficient of the studied samples in the range 0.5–1.2 THz was less than 20 cm$^{-1}$. The change in the refractive index of HgGa$_2$S$_4$ in the range 0.5–1.2 THz was 3.42–3.55 at $T$ = 300 K.