Concentration dependence of the band gap of Mn$_x$Fe$_{1-x}$In$_2$S$_4$ alloys on their composition

The transmittance spectra of MnIn$_2$S$_4$ and FeIn$_2$S$_4$ ternary compounds and Mn$_x$Fe$_{1-x}$In$_2$S$_4$ alloys in the fundamental absorption edge region are studied. The samples were grown by planar melt crystallization. From the experimental spectra, the band gaps of the compounds and their alloys are determined, and the concentration dependence of the alloy band gap on the component content is established. It is found that the band gap nonlinearly varies with the composition parameter x and can be described by a quadratic function.