Theoretical prediction of a novel hexagonal narrow-gap silicon allotrope under high pressures

Silicon material plays a vital role in contemporary technology-related fields, including electronics and the photovoltaics. There is a growing demand for exploring new silicon structures with potential applications, and numerous metastable structures have been reported. In this study, we present the prediction of a novel stable sp 3 hybridized silicon allotrope using particle swarm optimization global structure search. The predicted Si allotrope is a semiconductor with an indirect band gap of approximately 0.21 eV. It possesses three Si basis atoms in the unit cell, and we named it Si3. Interestingly, when subjected to strain, it undergoes a transition from a semiconductive state to a metallic state. Furthermore, moderate tensile strain enhances the interactions between silicon and lithium atoms, suggesting its potential for Li-ion batteries. Additionally, Si3 exhibits exceptional sunlight absorption across a wide range of wavelengths, with a significantly higher light absorption intensity than cubic diamond silicon. These findings have important implications for photovoltaic applications.