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Theoretical Study Of The Structural Phase Transition In Rbcaf3, L. L. Boyer, John R. Hardy

John R. Hardy Papers

We have made a first-principles study of the structural phase transition at T_c = 193 K in RbCaF₃, using interionic potentials derived by the Gordon-Kim approach, and a new extension of the quasiharmonic approximation for the free energy. The transition is caused by instability of a triply degenerate R-point vibration which leads to a coordinated rotation of the CaF₆ octahedra. We find that, as the lattice contracts, the quasiharmonic frequency of the R-point vibrations becomes imaginary at approximately 1280 K: Below this temperature the static lattice energy, as a function of CaF₆ rotation, has a …