Use of Computer Simulation Technique to Study Atomic Migration in Solids

摘要

Nature of ionic motions in superionic conductors AgI and Ag sub 2 S are studied using molecular-dynamics (MD) technique. Our model of these superionic conductors is based on the use of effective pair-potentials. To determine the constants in these potentials, cohesive energy and bulk modulus are used as input; in addition one uses notions of ionic size based on the known crystal structures. The alpha in equilibrium beta structural phase transition in AgI is studied with the use of the new molecular dynamics (NMD) technique which allows for a dynanical variation of the shape and size of the cell. In the present model, upon heating the beta -AgI, the iodine ions undergo a hcp implies bcc transformation and silver ions become mobile, whereas the reverse transformation is observed on cooling of alpha -AgI. The calculated alpha in equilibrium beta transition temperature, and structural and dynamical properties are in good agreement with experiments. It has also become possible to observe crystal growth of alpha and beta phases when a system of molten AgI is cooled to different temperatures. Transition temperatures determined from the studies of alpha in equilibrium beta structural transitions and from crystal growth studies are in good agreement. The calculated transition temperatures are also in good accord with the experimental results. A model of superionic conductor Ag sub 2 S is also studied. The silver density map and its temperature dependence is calculated. We have also calculated the x-ray and neutron structure factors which explain the anomalous intensity at and near Q = (1.6, 1, 0) observed in diffuse scattering experiments. A study of low-energy excitations in Ag sub 2 S is in progress. (ERA citation 11:011577)