Anharmonic effective pair potentials in CaTiO₃, SrTiO₃ and CaGeO₃ perovskite

The temperature dependence of EXAFS Debye–Waller factors in CaTiO₃, SrTiO₃ and CaGeO₃ perovskite was investigated with the cumulant expansion method. The CaGeO₃ perovskite as an analogue of the Earth's lower-mantle mineral was synthesized in a cubic anvil type apparatus under 10 GPa 1250 K. The Ca, Ti and Ge K-edge EXAFS spectra were measured in transmission mode at temperature up to 1100 K. The effective pair potentials, V(u) = αu²/2 + βu3/3!, were evaluated and the Grüneisen parameter were calculated. The potential coefficients α and β for Ti—O bond in CaTiO₃ are 6.9 eV Å⁻² and −38 eV Å⁻³, respectively. Those for Ge—O bond in CaGeO₃ are 9.8 eV Å⁻² and −36 eV Å⁻³, respectively.