Elastic, vibrational, and thermodynamic properties of MgGeO3 postperovskite investigated by first principles simulation

[1] Elastic, vibrational and thermodynamic properties of MgGeO3 perovskite (Pv) and postperovskite (PPv) were calculated based on the density functional first principles methods. We found that the calculated properties of MgGeO3 are quite similar to those of MgSiO3 in particular for anomalously large c66 and small c55 of PPv, but not fully comparable in particular for the velocity contrasts across the phase change, which are all negative in compressional, shear and bulk velocities at the transition pressure unlike the typical features of the D″ seismic discontinuity. MgGeO3 PPv has larger elastic anisotropy than MgSiO3, but their style is quite similar. Significant orientational preference is needed for PPv polycrystalline aggregate to reproduce seismic shear wave polarization anisotropy observed in D″ except for a case with the c direction aligned vertically. Lattice dynamics calculations indicate that the both phases are vibrationally stable under high pressure, and quasi-harmonic thermodynamics provides the Clapeyron slope of PPv phase boundary of ∼+7.8 MPa/K, which is quite comparable to that reported for MgSiO3. While the calculated Raman frequencies are in excellent agreement with experimental values in Pv, those are found less consistent in PPv.