Theoretical prediction of the high-pressure phase Ge-Cmca

Ab initio pseudopotential total energy density-functional theory--local-density approximation calculations were performed to study the crystalline structures of Ge under pressure. Following the well established sequence of structural phases (diamond→β-Sn→Imma→sh) under increasing pressure, we predict a transition into a new phase, with Cmca space-group symmetry, at 90±2 GPa. We estimate the superconducting transition temperature Tc for this phase to be in the range 2 to 7 K, the same range obtained previously by detailed calculations for the Ge-sh phase. The Cmca phase should remain stable up to 137±10 GPa where a transition to the hcp structure is predicted to occur. The same path is followed by Si although at lower pressures