Chemical bond approach to the structures of chalcogenide glasses with reversible switching properties

The structures of some chalcogenide glasses of interest due to their reversible threshold swithching (Si₁₈Ge₇As₃₅Te₄₀) and memory switching (Ge₁₅Te₈₁Sb₂S₂ and Ge₂₄Te₇₂Sb₂S₂) properties have been examined in terms of the chemical bonds expected to be present in these materials. Using simple considerations based on coordination numbers and bond energies, the average number of near neighbors of each type expected to surround a given central atom has been estimated. These average numbers of bonds have been used to estimate the cohesive energies of these glasses, assuming simple additivity of bond energies. The results of these calculations can be used to rationalize the observed switching properties of these materials, as well as to suggest criteria for selecting other materials with desirable properties.