Elasticity of MgO and a primary pressure scale to 55 GPa

We have studied the elasticity and pressure-density equation of state of Mg O in diamond cells to 55 GPa and have doubted the previous pressure limit o f accurate elasticity determinations for crystals. Integrating single-cryst al velocity data from Brillouin scattering measurements and density data fr om polycrystalline x-ray diffraction, we obtained the three principal elast ic tensor elements (C-11, C-12, and C-44) and various secondary elasticity parameters, including single-crystal elastic anisotropy, Cauchy relation, a ggregate sound velocities, and Poisson's ratio, as functions of pressure. T he present study also provides a direct determination of pressure without r ecourse to any prior pressure standard, thus creating a primary pressure sc ale. The commonly used ruby fluorescence pressure scale has thus been impro ved to 1% accuracy by the new MgO scale.