Use of computer simulation technique to study atomic migration in solids

Nature of ionic motions in superionic conductors AgI and Ag2S 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 α in equilibrium β 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 β-AgI, the iodine ions undergo a hcp → bcc transformation and silver ions become mobile, whereas the reverse transformation is observed on cooling of α-AgI. The calculated α in equilibrium β transition temperature, and structural and dynamical properties are in good agreement with experiments. It has also become possible to observe crystal growth of α and β phases when a system of molten AgI is cooled to different temperatures. Transition temperatures determined from the studies of α in equilibrium β 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 Ag2S 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 Ag2S is in progress