Metastable C-centered orthorhombic Si8 and Ge8

A theoretical prediction on the structural stabilities, mechanical properties, and electronic properties of the C-centered orthorhombic (Cco) Si8 and Ge8 is presented, inspired by a recently proposed carbon allotrope structure, Cco-C8. Energetically comparable with previously known metastable phases, Cco-Si8 and Cco-Ge8 may be obtained by decompressing the high-pressure β-Sn phases, or by compressing the corresponding nanotubes. The calculated bulk moduli of Cco-Si8 and Cco-Ge8 are close to those of the diamond phases. Further study of the electronic properties reveals that the band gaps of Cco-Si8 and Cco-Ge8 are tunable with variations in lattice parameters.