Ab initio structure of MgSiO₃ ilmenite at high pressure

The structural properties of MgSiO₃-ilmenite at high pressures are determined using Ab initio variable cell-shape molecular dynamics. Our athermal results at zero pressure are in excellent agreement with single-crystal measurements. The predicted lattice constants compare favorably with powder X-ray diffraction data. The internal parameters are shown to vary only slightly with pressure. The c axis is considerably more compressible than the a axis and our results suggest that this anisotropic behavior arises in the relatively larger compressibility of MgO₆ with respect to SiO₆ octahedra. Both octahedral types remain highly distorted with the degree of distortion decreasing (more rapidly in MgO₆) under compression. By comparing free energies, it is shown that MgSiO₃ should transform from the ilmenite to the perovskite structure at 30 GPa (for static lattice). At the transition, the density and elastic moduli increase substantially.