We use a flexible potential model to perform large-scale molecular dynamics simulations of Mg 2 SiO 4 melt up to pressures of 24 GPa and at temperatures between 2390 K and 3200 K. We find that thermal pressure and the Grüneisen parameter γ increase linearly with density, independent on temperature. γ increases from 0.5 at ambient pressure to 0.75 at 24 GPa. While Si stays overwhelmingly in tetrahedral coordination, the coordination of Mg increases significantly under compression, from an average 5-fold coordination at room pressure to 7-fold coordination at 24 GPa. Medium range order in Mg 2 SiO 4 as expressed by X-ray and neutron structure factors change considerably with pressure, with features at low wave-vectors q sharpening considerably and shifting to higher q. Diffusivity of the atomic species in Mg 2 SiO 4 decrease uniformly with pressure and are well described by an Arrhenius law. For viscosity η we find good agreement with experiments at room pressure, and predict a rapid increase with pressure.

We use a flexible potential model to perform large-scale molecular dynamics simulations on self-diffusivity and viscosity of Mg 2 SiO 4 melt up to pressures of 32 GPa and over a temperature range of 2600 to 3200 K. We find that self-diffusivity decreases and viscosity increases uniformly with pressure, the latter from values of 10

We report first-principles phase diagram calculations for the binary systems HfC-TiC, TiC-ZrC, and HfCZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiC-ZrC and HfC-TiC, with consolute temperatures in excess of 2000 K. The HfC-ZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ϳ7%, ϳ20%, and ϳ0%, for HfC-TiC, TiC-ZrC, and HfC-ZrC, respectively. Predicted miscibility gaps are symmetric for HfC-ZrC, almost symmetric for HfC-TiC and asymmetric for TiC-ZrC.