Parameterized Empirical Potentials For BaF2 From Ab-Initio Methods Applied To The Study Of Superionic Transition Of C-BaF2

Abstract

Potentials for BaF2 have been generated from the ab-initio methods and tested to determine its applicability in classical dynamics. The minimization process for this force field was stopped at 17% where forces, stresses and energies were close enough to the ab-initio results. Using the steepest descent approach, this potential was used to calculate the lattice constant of BaF2 and inter-atomic bond length. The lattice parameter for BaF2 using this force field was found to be 6.13 Å comparable to 6.10 Å from DFT. Other parameters that are compared are the bond length between cations and anions and transition temperatures. The results are found to be in good agreement with the already computed results from DFT. The melting temperature calculated at 1700 K is closer to 1641 K from experiment than the already reported. This current potential therefore seems to be an improvement to the classical potentials that have previously been used in molecular dynamics study of BaF2. Whereas type I superionics evolve to highly conducting state abruptly, type II evolve continuously [2]. Specific heat jump and high-temperature conductivity behaviour for superionics are due to diffuse phase transition at which there is a relative disordering of the anion sub lattice. Presence of a broad peak in the heat capacity curve has been observed at a temperature TC (specific heat maximum) essentially below the melting point Tm for most of superioncs [8]. Earlier studies based on empirical potentials have focused on MgF2, CaF2, SrF2 and BaF2 [3-7]. Calculations are done using classical molecular dynamics of ref [9] has shown that BaF2 becomes superionic at about 1250 K. The rest of this paper is given such that section II is presentation of the methods used in obtaining the potential from ab initio methods and minimizing the force field, section III is the results and discussions then conclusions in section IV.