First-principles theory of phase transformations in ionic solids

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Abstract
A unified theory of melting, superionicity, and soft-mode displacive transitions is obtained from the application of Gordon-Kim calculated pair potentials in quasiharmonic equation-of-state calculations for alkali halides, alkaline earth halides and fluoperovskite materials. The method requires no experimental input other than universal constants. In each case the phase transition is found to be initiated by an instability in the lower temperature phase. Melting is predicted to occur when the vibrational pressure is too large to be balanced by the static pressure. Superionicity in the fluorites appears to be related to unstable vibrations with X3 symmetry. The structural transition at -193K in RbCaF3 is viewed as an unfreezing of the low temperature structure. This occurs when the free energy of the low temperature phase no longer possesses a minimum for finite static values of the coordinated “rotation” (R25 symmetry) of the CaF6 octahedra.