Optical and electrical properties of Tl-S glasses

The frequency and temperature dependence of ac conductivity and optical absorption have been measured for four Tl-S glasses, TlS, Tl₂S₃, TlS₂ and Tl₂S₅, prepared by a melt-quenching method. The ac conductivity has been measured over a frequency range 0.1 Hz to 1.8 GHz and a temperature range 190-273 K. The optical absorption was measured at room temperature over a wavelength range 200-2600 nm. We have determined the electrical and optical band gaps from the experimental results. For each glass, the electrical band gap is larger than the optical band gap and the difference increases with increasing sulfur concentration. The frequency dependence of ac conductivity varies with composition of the glasses. We suggest that these results are due to an increase of localized states in the band gap with increasing sulfur concentration.