Optical absorption tails and structural disorder in Sn(x)Ge (1-x)Se(2.5) and other chalcogenide alloy glasses and liquids.

Urbach slopes siguna of absorption edges and glass transition temperatures T(g) in pseudobinary chalcogenide alloy glasses with 0 - X - 0.6 containing predominantly tetrahedral building blocks have been studied as the resistance to shear of the building blocks is varied through the critical composition x = 0.4. The composition dependences of siguna and T(g) are nearly parallel, with softening occurring above x = 0.4, in agreement with theoretical expectations. Chemical trends in Urbach slopes of other chalcogenide alloy glasses and liquids are discussed within the framework of constraint theory and a simple statistical model.