Ab-initio study of the Imma phase in silicon

We present an ab-initio study of the high-pressure phase transitions through the structure sequence beta-tin->Imma->sh of silicon. The plane-wave pseudopotential approach to the density-functional theory within the generalized-gradient approximation implemented in VASP is used. We determine the equilibrium properties of each structure and the critical values of the phase transition under hydrostatic pressure. We observe a continuous phase transition from the beta-tin to the Imma phase and a discontinuous one from the Imma to the sh phase.