A calculation of the vibrational entropy of anion Frenkel defects in fluorite crystals

The vibrational entropies associated with an anion Frenkel defect in CaF2, SrF2 and BaF2 have been calculated using an extension of the method of Harding and Stoneham (1981). A feature of the calculation is that the vacancy and interstitial sites are handled together, not one at a time. This ensures charge neutrality and enables effects of Coulomb interactions to be computed by the usual Ewald, method. The results are compared with measured data. A by-product of the computation is an estimate of a term occurring in the calculations of the formation volume which had been deleted previously (Gillan 1981).