Theoretical prediction of the high-pressure phase Ge-Cmca

Ab initio pseudopotential total energy density-functional theory-local-dens ity approximation calculations were performed to study the crystalline stru ctures of Ge under pressure. Following the well established sequence of str uctural phases (diamond --> beta -Sn --> Imma --> sh) under increasing pres sure, we predict a transition into a new phase, with Cmca space-group symme try, at 90 +/- 2 GPa. We estimate the superconducting transition temperatur e T-c for this phase to be in the range 2 to 7 K, the same range obtained p reviously by detailed calculations for the Ge-sh phase. The Cmca phase shou ld remain stable up to 137 +/- 10 GPa where a transition to the hcp structu re is predicted to occur. The same path is followed by Si although at lower pressures.