The entropy of formation of an anion frenkel defect in calcium fluoride

Abstract

The vibrational entropy change at constant volume that is associated with the formation of anion Frenkel defect in calcium fluoride has been calculated in the quasiharmonic approximation by finding the vibrational frequencies at q = 0 of an electrically neutral 32-molecule “supercell” containing both an anion vacancy and an anion interstitial. The supercell is of a size and shape such that every ion that is a first or second neighbour to a defect can be given the position it occupies in the displaced, defect lattice. From these results, and the calculated energy of formation of a Frenkel defect at constant volume, we have calculated the enthalpy, entropy and volume changes associated with Frenkel defect formation in calcium fluoride at constant pressure.