On the band-gap width of (FeIn$_2$S$_4$)$_{1-x}$(In$_2$S$_3$)$_x$ alloys

The transmittance spectra in the region of the fundamental absorption edge are studied at 80 and 295 K for the single-crystal compounds FeIn$_2$S$_4$ and In$_2$S$_3$ and (FeIn$_2$S$_4$)$_{1-x}$(In$_2$S$_3$)$_x$ alloys grown by the Bridgman method. From the experimental spectra, the band gap of the FeIn$_2$S$_4$ and In$_2$S$_3$ compounds and (FeIn$_2$S$_4$)$_{1-x}$(In$_2$S$_3$)$_x$ alloys is determined. The dependence of the band gap on the composition parameter x of the alloy is constructed. It is established that the band gap nonlinearly varies with x and can be described by a quadratic dependence.