Experimental study of the crystal stability and equation of state of Si to 248 GPa

The room-temperature equation of state of silicon, as determined from in situ energy-dispersive x-ray diffraction using a synchrotron source, is presented to 248 GPa. An intermediate phase between the primitive hexagonal and hexagonal-close-packed (hcp) phases is stable from 37.6\ifmmode\pm\else\textpm\fi{}1.6 to 41.8\ifmmode\pm\else\textpm\fi{}0.5 GPa, and is shown to be the same structure as the X phase of the alloy ${\mathrm{Bi}}_{0.8}$${\mathrm{Pb}}_{0.2}$, and is not a simple restacking of hexagonal-close-packed layers (i.e., Sm-type, dhcp, or thcp). The hcp\ensuremath{\rightarrow}fcc phase transition occurs at 79\ifmmode\pm\else\textpm\fi{}2 GPa. The fcc phase remains stable to 248 GPa, where the silicon fractional volume is V/${\mathit{V}}_{0}$=0.361\ifmmode\pm\else\textpm\fi{}0.006. Excellent agreement between first-principles total-energy calculations and these results is observed for the hcp\ensuremath{\rightarrow}fcc transition pressure, and the fcc Si pressure-versus-volume equation of state.