Theoretical study of high-pressure orthorhombic silicon

Ab initio calculations of the structural properties of a recently discovered body-centered orthorhombic phase of Si are presented. This high-pressure phase (denoted Imma after its space group) was observed at pressures between the transition pressures for the β-Sn and simple hexagonal (sh) phases of Si. Total-energy calculations indicate that the optimal Imma structure varies with cell volume from the β-Sn structure at large volumes to the sh structure at small volumes when all three structures are described in terms of the Imma unit cell. The energy of the Imma phase is found to be lower than or equal to the energies of the β-Sn and sh phases for all cell volumes.