Structural and lattice dynamical properties of ionic solid with rutile and perovskite structure

Abstract

This work is mainly concerned with the application of classical interatomic potentials to modeling the properties of ionic solids in general. The theoretical model employed herein to represent energy surfaces in ionic systems consists of contral pairwise interactions between rigid ions with fractronal charges. The functional form of the pair potential is based on a soft sphere model in which the adjustable parameters are determmed by an empirical fitting procedure. The first prime objective of the work is to show which performance one can expect from this simple method, when it is applied properly and skillfully. Specifically, this approach is used to describe and predict a range of the structural and lattice dynamical properties of $MgF_2$ and $TiO_2$ with rutile structure, which is of particular interest in both geophysical and technological fields and also useful from a computational point of view. For magnesium difluoride ($MgF_2$), a two-body potential model is developed from the known crystal structure at low temperature and athermal-limit values of the elastic constants to eliminate the thermal effect on the potential function. The reliability of this potential model is critically evaluated through the calculations of structural and lattice dynamical properties of $MgF_2$. Static energy calculations are extensively performed to predict the lattice parameters and elastic properties of $MgF_2$ both at ambient and elevated pressure conditions. The calculated static equilibrium structure corresponding to the energy minimum is in excellent agreement with the experimental results obtained at 52 K. The static compression curve in rutile phase of $MgF_2$ is presented and the bulk modulus $B_0$ and its pressure derivative $B_0^\prime$ at zero pressure, which are often used to define the compression curve, are in satisfactory agreement with available roomtemperature data. From a calculation of the variation of the lattice parameters with pressure, the linear compr...