High-Pressure Phase Diagram of Silicon*

Constructing accurate high-pressure phase diagrams has been one of the goals in high-energy-density sciences. The impetus for this work stemmed from the fact that silicon is of great importance in the design of inertial confinement fusion ablators and geophysics. We have employed an evolutionary algorithm-based structure-searching method, in conjunction with density-functional-theory calculations, to determine the structure for a given pressure and temperature. Previously validated stable crystalline phases—cd, bct, sh, hcp, and fcc—have been charted along with the associated triple points at pressures of up to 3.5 TPa. Once the cold curve was obtained, first‑principles lattice-dynamical and MD calculations were used to identify the phase transition boundaries and melting line based on Gibbs free energy at finite temperatures. This study has identified a new dhcp structure sandwiched in between the sh and hcp phases (34 to 40 GPa) and a fcc-to-bcc structural transition in the region of ~2.7 TPa. We will present these results with discussions about the thermodynamic and electronic properties of silicon in each identified phase.