First-principles lattice dynamics and thermoelasticity of MgSiO₃ ilmenite at high pressure

Phonon dispersions of MgSiO₃ ilmenite, a major mantle mineral, are calculated at high pressures by using first-principles density functional perturbation theory. Our results for the zone center modes and their initial pressure dependencies agree very well with the experimental Raman data. However, comparison with infrared data shows some discrepancy. The calculated eigenvectors indicate that low-frequency modes are primarily associated with the stretching of weak Mg-O bonds, whereas high-frequency modes are dominated by the stretching of stiff Si-O bonds. All the mode frequencies are shown to increase monotonically with increasing pressure and no soft modes exist over the pressure regime studied (-11 to 33 GPa). Several thermodynamic properties including the thermal equation of state, thermal expansivity, entropy, and heat capacity that can be useful to better understand the contribution of the silicate ilmenite phase to the composition and dynamics of the Earth's mantle are successfully presented at geophysically relevant pressure-temperature conditions.