(FeIn$_{2}$S$_{4}$)$_{x}$ $\cdot$ (AgIn$_{5}$S$_{8}$)$_{1-x}$: Crystal structure, nuclear $\gamma$-Resonance spectra, and band gap

(FeIn$_{2}$S$_{4}$)$_{x}$ $\cdot$ (AgIn$_{5}$S$_{8})_{1-x}$ alloy single crystals are grown in the entire region of composition parameters x by the Bridgman method. It is established that the single crystals crystallize in the cubic spinel structure. Nuclear $\gamma$-resonance spectroscopy is used to study the local configurations of iron ions in the structure of (FeIn$_{2}$S$_{4}$)$_{x}$ $\cdot$ (AgIn$_{5}$S$_{8})_{1-x}$ alloys. From the transmittance spectra in the region of the fundamental absorption edge, the band gap $(E_g)$ of the alloys is determined, and its concentration dependence is constructed. It is shown that $E_g$ nonlinearly varies with $x$.