Computational prediction of a new metallic silicon allotrope

Silicon, a crucial element in modern technological advancements, significantly contributes to various sectors such as electronics and solar energy conversion systems. The need to investigate novel silicon materials with promising applications is on the rise, leading to the identification of various new silicon allotropes. By first-principles calculations, we identify a new silicon phase in $R\bar{3}$ ($C_{3i}^2$) symmetry, which has a rhombohedral primitive cell with 7 silicon atoms, thus termed as R-Si7. The dynamic and thermal stabilities of R-Si7 have been verified by phonon mode analysis and ab initio molecular dynamics simulations, respectively. The mechanic properties of this new silicon allotrope has also been investigated. The X-ray diffraction patterns have been simulated and provided more structural information of R-Si7. Besides, the calculated electronic band structures and projected density of states show that R-Si7 is a metallic silicon material, and its conductivity mainly stems from the $p$-orbital electrons of silicon. These results pave the way for further exploration of this new silicon phase and its outstanding properties.