STRUCTURE AND ELASTICITY OF MGO AT HIGH-PRESSURE

The structural and elastic properties of MgO periclase were studied up to 150 GPa with the first-principles pseudopotential method within th e local density approximation. The calculated lattice constant of the B1 phase over the pressure range studied is within 1% of experimental values. The observed B1 phase of MgO was found to be stable up to 450 GPa, precluding the B1-B2 phase transition within the lower mantle. Th e calculated transition pressure is less than one-half of the previous pseudopotential prediction but is very close to the linearized augmen ted plane-wave result. All three independent elastic constants, c(11), c(12), and c(44) for the B1 phase are calculated from direct computat ion of stresses generated by small strains. The calculated zero-pressu re values of the elastic moduli and wave velocities and their initial pressure dependence are in excellent agreement with experiments. MgO w as found to be highly anisotropic in its elastic properties, with the magnitude of the anisotropy first decreasing between 0 and 15 Gpa and then increasing from 15 to 150 GPa. Longitudinal and shear-wave veloci ties were found to vary by 23 and 59%, respectively, with propagation direction at 150 Gpa. The character of the anisotropy changes qualitat ively with pressure. At zero pressure longitudinal and shear-wave prop agations are fastest along [111] and [100], respectively, whereas abov e 15 GPa, the corresponding fast directions are [100] and [110]. The C auchy condition was found to be strongly violated in MgO, reflecting t he importance of noncentral many-body forces.