Theoretical prediction of new C-Si alloys in C2/m-20 structure

We study structural, mechanical, and electronic properties of C₂₀, Si₂₀ and their alloys (C₁₆Si₄, C₁₂Si₈, C₈Si₁₂, and C₄Si₁₆) in C2/m structure by using density functional theory (DFT) based on first-principles calculations. The obtained elastic constants and the phonon spectra reveal mechanical and dynamic stability. The calculated formation enthalpy shows that the C-Si alloys might exist at a specified high temperature scale. The ratio of B/G and Poisson's ratio indicate that these C-Si alloys in C2/m-20 structure are all brittle. The elastic anisotropic properties derived by bulk modulus and shear modulus show slight anisotropy. In addition, the band structures and density of states are also depicted, which reveal that C₂₀, C₁₆Si₄, and Si₂₀ are indirect band gap semiconductors, while C₈Si₁₂ and C₄Si₁₆ are semi-metallic alloys. Notably, a direct band gap semiconductor (C₁₂Si₈) is obtained by doping two indirect band gap semiconductors (C₂₀ and Si₂₀).