Anharmonic and Anomalous Trends in the High-Pressure Phase Diagram of Silicon

A multifacetedfirst-principles approach utilizing density functional theory, evolutionary algorithms, and lattice dynamics was used to construct the phase diagram of silicon up to 4 TPa and 26 000 K. These calculations predicted that (i) an anomalous sequence of face-centered cubic to body-centered cubic to simple cubic crystalline phase transitions occur at pressures of 2.87 and 3.89 TPa, respectively, along the cold curve, (ii) the orthorhombic phases of Imma and Cmce-16 appear on the phase diagram only when the anharmonic contribution to the Gibbs free energy is taken into account, and (iii) a substantial change in the slope of the principal Hugoniot is observed if the anharmonic free energy of the cubic diamond phase is considered.