Integral-equation approach to medium-range order in molten and glassy chalcogenides

The first sharp diffraction peak (FSDP), a signature of medium-range order in molten and vitreous chalcogenides, is investigated using the integral-equation method. A variety of interatomic potentials, including uncharged and charged hard spheres, and realistic two-body interactions, are used in this study. The two-body potential consists of steric repulsions, Coulomb interactions due to charge-transfer effects, and the effects of electronic polarizability of ions. ${\mathrm{GeSe}}_2$ is treated as a specific example. The FSDP is observed in both the charged-hard-sphere and the realistic two-body interaction models. In both models steric and charge-transfer effects are found to give rise to Ge(${\mathrm{Se}}_{1/2}$ ${)}_4$ tetrahedra whose packing determines the medium-range order and the attendant FSDP. From the charge-charge structure factor it is found that the FSDP arises from spatial correlations where charge neutrality prevails. The nature of the medium-range correlation is elucidated through the temperature and density dependence of the FSDP and the thermal expansion determined from the shifts of the peaks in the static structure factor. We have also studied the effects of the potential parameters on the position of the FSDP. The results of the hypernetted-chain theory are compared with the molecular-dynamics results.