Optical properties at the absorption edge ofPt0.97S2

Optical properties of ${\mathrm{Pt}}_{0.97}$ ${\mathrm{S}}_2$ have been studied near the fundamental absorption edge of the semiconductor. Measurements of the index of refraction and of the absorption coefficient are reported for $\overrightarrow{\mathrm{E}}$ perpendicular and parallel to the $\overrightarrow{\mathrm{c}}$ axis, and as a function of the temperature. A qualitative band-structure model based on the ligand-field theory is presented. The first electronic transitions are found to be indirect for both polarizations and then are followed by a direct transition mainly allowed for $\overrightarrow{\mathrm{E}}$ parallel to the $\overrightarrow{\mathrm{c}}$ axis. The character of these transitions is discussed using the proposed band scheme.