First-principles calculation of the equation-of-state, stability, and polar optic modes of CaSiO₃ perovskite

We report the results of a first-principles LAPW calculation of the equation-of-state, dynamic stability, and infrared-active transverse optic vibrational mode frequencies of CaSiO₃ perovskite. A Birch-Murnaghan fit to the computed energy-volume relation of the cubic phase yields values of V_o = 45.62 ų, K_o = 227 GPa, and K_o′ = 4.29 for the thermally-corrected equation-of-state parameters. These values are in excellent agreement with recent quasi-hydrostatic compression data to 10 GPa, but significantly differ from values derived from higher pressure non-hydrostatic compression data. We calculate the volume dependence of the infrared-active TO mode frequencies using a frozen-phonon approach. The lowest frequency ferroic mode is predicted to occur near 228 cm^-1 at ambient pressure and displays classic soft-mode behavior in the tensile regime, in quantitative agreement with earlier molecular and lattice dynamical calculations, based on empirical potentials. These established a link between the low frequency ferroic mode and the thermally activated crystalline-amorphous transition in a model CaSiO₃ perovskite system. Our present calculations also reveal that the static cubic perovskite structure is unstable with respect to small octahedral rotations, corresponding to Brillouin zone edge dynamical instabilities, at ambient pressure and throughout the pressure range of the lower mantle. We speculate that coupling of the low frequency ferroic mode with octahedral tilting modes and strain lower the activation energy for the crystalline-amorphous transition.