Integral-equation theory of the origin of medium-range order in molten and vitreous chalcogenides

The origin of medium-range order in molten and vitreous GeSe2, signified by the first sharp diffraction peak (FSDP), is examined in terms of interatomic potentials using the integral-equation method. A systematic study of steric and charge-transfer effects shows that the FSDP originates from a combination of the excluded-volume effect and local charge neutrality. These effects are also found to be responsible for a network structure consisting of Ge(Se12/)4 tetrahedra. The anomalous temperature dependence of the FSDP can be ascribed entirely to an increase in the density of the system on cooling.