Unified Study of the Lattice Mechanics of Fluorites Indications of the Superionic Phase Transition

Abstract

A unified study of the different lattice mechanical properties of the three fluorites CaF,, SrF,, and BaF, is presented, using a semi-microscopic model and a single set of parameters. The cohesive energy, elastic constants, dielectric constants, and phonon dispersion curves are calculated. The temperature dependence of the phonon frequencies is estimated from a simple quasi-harmonic approach which indicates softening of a (100) zone boundary phonon mode which may lead to the diffuse transition to the superionic state.Verschiedene gittermechanjsche Ejgenschaften der drei Fluorite CaF,, SrF, und BaF, werden anhand eines halbmikroskopischen Modells unter Verwendung eines einzigen Parametersatzes untersucht. Kohasionsenergie, elastische Konstanten, dielektrische Konstanten und Phononendispersionskurven werden berechnet. Die Temperaturabhangigkeit der Phononenfrequenzen wird in quasiharmonischer Naherung abgeschatzt, dabei..wird die Erweichung einer (100)-Zonengrenzenphononenmode festgestellt, was zu dem diffusen Ubergang zum superionischen Zustand fiihren kann.

Reply to “Comment on ‘High-pressure phases of group-II difluorides: Polymorphism and superionicity’ ”

“…SrF 2 and BaF 2 are examined in Refs. [30,31,34]; as is the case for α-CaF 2 [17], a critical softening of a zone-boundary phonon at X is seen at increasing volumes. Phonon frequencies in stoichiometric Li 3 OCl have been discussed in detail in Ref.…”

“…Previous studies have used the QHA and/or phonon modes to explain superionic behaviour in SrF 2 , BaF 2 , Li 3 OCl and AgI [30][31][32][33]. SrF 2 and BaF 2 are examined in Refs.…”