Abstract
After performing an analysis of the anomalous concentration and temperature dependence of absorption tails in GexSe1-x and AsxSe1-x systems, the author proposes that these glasses retain locally layered structures at particular stoichiometries corresponding to GeSe2 and As2Se3, and that a reversible structural change is taking place well below the glass transition temperature. A phenomenological model of layer buckling is presented to explain the slope of the exponential tail in the optical absorption of glasses, and this is analogous to the Urbach rule for crystalline materials.