Ab initio study of MgSiO₃ low-clinoenstatite at high pressure

We investigated the athermal high-pressure behavior of the structural properties of MgSiO₃ low clinoenstatite using the plane wave pseudopotential method within the local density approximation. The experimental zero pressure structure and pressure variations of the lattice parameters were reasonably well reproduced. The calculated atomic positions vary slightly and monotonically with pressure. Our results showed that MgO₆ octahedra are three times more compressible than SiO₄ tetrahedra, consistent with the general observation that the SiO₄ tetrahedra are nearly incompressible in silicates. Mg2O₆ octahedra (the larger of two Mg sites) remain most distorted over the pressure regime studied while Mg1O₆ octahedra and both types of SiO₄ tetrahedra show similar level of distortions. The tetrahedral chain angles were shown to slightly vary with pressure indicating that the structure under compression always remains highly distorted away from the ideal close-packing of O atoms.