Structural phase transitions in Si under hydrostatic and uniaxial compression

Abstract

Phase transitions induced by hydrostatic and uniaxial compression of Si are examined using a density functional--based metadynamics method. A metastable Cmcm structure is predicted to form under hydrostatic compression at 300 K in the pressure range near 20 GPa. Enthalpy calculations indicate that this new phase of Si is energetically competitive with the other Si structures found in this pressure range. This phase is shown to be dynamically stable, with a structure closely related to that of the \ensuremath{\beta}-Sn and Imma structures previously identified in experiments. Uniaxial compression at 15 GPa and 300 K obtained simple hexagonal Imma and \ensuremath{\beta}-Sn structures. The low energy barrier for the formation of the Cmcm structure from the diamond ambient pressure form suggests that this structure may be seen in experiment with the proper experimental conditions.