A new metastable phase of silicon in the \textit{Ibam} structure

In a study aimed at finding new useful forms of silicon, we use an \textit{ab initio} random structure searching (AIRSS) method to identify a new phase of silicon in the \textit{Ibam} structure. The \textit{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 \textit{Ibam} structure as well as all other known Si structures, including the previously predicted phases st12 and bct. These results indicate that \textit{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.