Energy landscape and photoinduced structural changes in chalcogenide glasses

It is shown that the magnitude of photostructural changes in chalcogenide glass is a function of the connectivity and fragility of the glass network. Glasses with a floppy network undergo larger photostructural changes, as shown by photorelaxation, photoexpansion and photodarkening measurements. The increasing magnitude of the changes correlates with a decrease in coordination number . The phenomenon is linked to the topography of the energy landscape and to the fragility of samples with different values. Since a high density of minima on the landscape is generally associated with a floppy network and fragile glassformers, it is suggested that photoexcitation provides a means for the system to explore these minima and that, consequently, fragile systems exhibit far larger photostructural changes, as observed. On the other hand, strong systems with optimal rigidity undergo very little structural change upon irradiation. This is attributed to the low density of configurational states on the landscape. A parallel between individual bond breaking in the bond lattice model and individual photoexcitation of bonding electrons during irradiation is made to support the use of the landscape formalism in describing photostructural changes.