Temperature dependence of the band gap of FeS2

Optical-absorption studies of the band gap $E_g$ in a 150-μm-thick natural single crystal of Fe${\mathrm{S}}_2$ for temperatures $T$ up to 425 K are presented. Different contributions to the temperature dependence of $E_g$ are discussed. It is argued that the variation of the form $E_g\mathrm{}\left(T\right)=E_g\mathrm{}\left(0\right)+aT+b{T}^2$, with the parameters $E_g\mathrm{}\left(0\right)=0.835\mathrm{}$ eV, $a=4.0\mathrm{}\times {10}^{-5\mathrm{}}$ eV/K, $b=-7.4\mathrm{}\times {10}^{-7\mathrm{}}$ eV/${\mathrm{K}}^2$ obtained from the best fit, provides a good physical interpretation of the data.