The longitudinal acoustic and optical phonon branches along the $\Gamma \mathrm{\text{−}}X$ direction of MgO at 35 GPa have been determined by inelastic x-ray scattering using synchrotron radiation and a diamond-anvil cell. The experimentally observed phonon branches are in remarkable agreement with ab initio lattice dynamics results. The derived thermodynamic properties, such as the specific heat $C_V$ and the entropy $S$ are in very good accord with values obtained from a thermodynamically assessed data set involving measured data on molar volume, heat capacity at constant pressure, bulk modulus and thermal expansion.

• Received 27 February 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.035507