Route to a direct-gap silicon allotrope Si 32 .

Using swarm-intelligence-based structure prediction methods, we predict a novel direct bandgap silicon allotrope with open channels at ambient conditions. This silicon phase, termed Si 32 , can be produced by removing Sr atoms from a new Cmcm -SrSi 8 clathrate-like compound, which is calculated to be thermodynamically stable under epitaxial strain at high pressures. Si 32 is predicted to have a direct bandgap of ∼1.15 eV and exceptional optical properties. The prediction of novel silicon clathrate-like structure paves the way for the exploration of novel silicon phases with extensive application possibilities.