Temperature dependence of the band gap of the single-crystal compounds In$_{2}$S$_{3}$ and AgIn$_{5}$S$_{8}$

For the single-crystal compounds In$_{2}$S$_{3}$ and AgIn$_{5}$S$_{8}$ produced by chemical gas-transport reactions and the Bridgman method (vertical version), the transmission spectra in the region of the fundamental absorption edge are studied in the temperature range from 20 to 300 K. From the recorded spectra, the band gaps of the In$_{2}$S$_{3}$ and AgIn$_{5}$S$_{8}$ single crystals are determined and the temperature dependences of the band gaps are constructed. It is established that, as the temperature is lowered, the band gap increases for both of the compounds. Calculation of the temperature dependences is performed. It is shown that the calculated and experimental values are in agreement with each other.