UNIFIED STUDY OF THE LATTICE MECHANICS OF FLUORITES - INDICATIONS OF THE SUPERIONIC PHASE-TRANSITION

Unified study of the different lattice mechanical properties of the three fluorites CaF2, SrF2, and BaF2 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.