The optical absorption study of synthetic RuS2 single crystals grown in tellurium fluxes is reported over a temperature range from 4.2 K to 300 K. The data are fitted to several expressions for the shape of the absorption edge, with the best fit being to that expected for indirect allowed transitions. The indirect band gaps at various temperatures are determined. The temperature dependence is analysed by the Varshni equation and an empirical expression proposed by O'Donnell and Chen. The parameters that describe the temperature dependence of the energy gap of RuS2 are evaluated. At temperatures lower than 100 K, several features which occur below the absorption edge are superposed on the absorption curve. The origin of these features is discussed.
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