On the Urbach rule in non-crystalline solids

A comparative analysis of temperature behavior of optical absorption edge is performed for non-crystalline materials where the Urbach behavior is observed (SiO2), with deviations from the Urbach behavior (20PbO · 80SiO2) and with a mixed behavior (As2S3). For glassy As2S3, the Urbach behavior of the absorption edge in a limited temperature range is explained from the point of view of two components of structural disordering – static and dynamic. A parallel red shift of the absorption edge in As2S3 and, consequently, the temperature-independent Urbach energy at T < 300 K, are related to the lack of medium-range order in the atomic arrangement in the presence of short-range order.