Prediction of a metastable phase of silicon in the Ibam structure

In a study aimed at finding new useful forms of silicon, we use an ab initio random structure searching (AIRSS) method to identify a new phase of silicon in the Ibam structure. The Ibam phase is found to be semimetallic within density functional theory with a small band overlap, and it is expected that quasiparticle corrections using the GW approximation would yield a semiconducting state with a small band gap. Calculation of the lattice dynamics reveals that the structure is locally stable. Enthalpy-pressure relations are calculated for the Ibam structure as well as all other known Si structures, including the previously predicted phases st12 and bct. These results indicate that Ibam silicon is metastable over the pressure range considered. Calculated coexistence pressures of the other known phase transitions are in good agreement with experimental observation.