PREDICTION OF AN ORTHORHOMBIC PHASE OF GERMANIUM

Based on ab initio total energy calculations, a body-centered orthorhombic structure (denoted Imma after its space group) is predicted to be a stable high-pressure phase of Ge for pressures between the transition pressures of the beta-Sn and the simple hexagonal (sh) phases. This structure was recently observed in Si for pressures between the same two transitions. The energy of the Imma phase of Ge is found to be lower than or equal to the energy of the other two phases for all unit cell volumes. The structure of the Imma phase is shown to vary continuously with cell volume from a beta-Sn-like structure at large volumes to the sh structure at small volumes, when all three structures are described in terms of the Imma unit cell.