Single-crystal elasticity of MgO at high pressure

The single-crystal elastic properties of MgO were measured by Brillouin sca ttering in diamond anvil cell to 18.6 GPa. These measurements demonstrate t hat quasihydrostatic stress conditions can be obtained in solid pressure me dium by thermal annealing, and establish the accuracy with which elastic mo duli can be measured over this extensive pressure range (+/- 5% in dC(ij)/d P). The acoustic anisotropy of MgO decreases with pressure, with MgO becomi ng isotropic at similar to 21.5 GPa. Deviations from noncentral interatomic forces, as measured by the Cauchy relations, increase steadily with pressu re.