TEMPERATURE-DEPENDENCE OF THE PHOTOINDUCED ANISOTROPY IN CHALCOGENIDEGLASSES - ACTIVATION-ENERGIES AND THEIR INTERPRETATION

Three distinct regions have been observed in the measured temperature dependence of photoinduced anisotropy in chalcogenide glasses (Se and AsSe). At high temperatures the relaxation of the induced anisotropy i s governed by an Arrhenius law with a change of activation energy at t he glass-transition temperature T-g (thus suggesting a method for dete ction of T-g in glasses). Conversely, the low-temperature relaxation i s not thermally activated. These observations are explained by introdu cing a triple-well configurational diagram for the valence-alternation pairs (VAP's) and their environments, which are believed to be respon sible. A transient neutral state connects two (initial and final) char ged states of the VAP's. A symmetry reversal of the pyramid centered a t an overcoordinated chalcogen atom is suggested as a microscopic mech anism for the photoinduced anisotropy.