Defect entropies and enthalpies in Barium Fluoride

Abstract

Various experimental techniques, have revealed that the predominant intrinsic point defects in BaF$_2$ are anion Frenkel defects. Their formation enthalpy and entropy as well as the corresponding parameters for the fluorine vacancy and fluorine interstitial motion have been determined. In addition, low temperature dielectric relaxation measurements in BaF$_2$ doped with uranium leads to the parameters {\tau}$_0$, E in the Arrhenius relation {\tau}={\tau}$_0$exp(E/kBT) for the relaxation time {\tau}. For the relaxation peak associated with a single tetravalent uranium, the migration entropy deduced from the pre-exponential factor {\tau}$_0$, is smaller than the anion Frenkel defect formation entropy by almost two orders of magnitude. We show that, despite their great variation, the defect entropies and enthalpies are interconnected through a model based on anharmonic properties of the bulk material that have been recently studied by employing density-functional theory and density-functional perturbation theory.