Ab initio structure of MgSiO3 ilmenite at high pressure

The structural properties of MgSiO3-ilmenite at high pressures are determin ed using Ab initio variable cell-shape molecular dynamics. Our athermal res ults at zero pressure are in excellent agree ment with single-crystal measu rements. The predicted lattice constants compare favorably with powder X-ra y diffraction data. The internal parameters are shown to vary only slightly with pressure. The c axis is considerably more compressible than the a axi s and our results suggest that this anisotropic behavior arises in the rela tively larger compressibility of MgO6 with respect to SiO6 octahedra. Both octahedral types remain highly distorted with the degree of distortion decr easing (more rapidly in MgO6) under compression. By comparing free energies , it is shown that MgSiO3 should transform from the ilmenite to the perovsk ite structure at 30 GPa (for static lattice). At the transition, the densit y and elastic moduli increase substantially.