Effect of Vacancy on Physical Properties of Cmcm Si24

Abstract

Researchers recently synthesized a new orthorhombic allotrope of silicon, namely Cmcm Si₂₄. This work studied the effect of one vacancy for Cmcm Si₂₄ on structural, electronic, and mechanical anisotropy properties by the first principle calculation. The lattice parameters of Cmcm Si₂₄:V are the same as Cmcm Si₂₄, but the density of Cmcm Si₂₄:V is larger than that of Cmcm Si₂₄. Moreover, the values of elastic moduli for Cmcm Si₂₄:V are smaller than for Cmcm Si₂₄. The Cmcm Si₂₄:V has mechanical stability and anisotropy and has lower formation energy than diamond-Si. The analysis of the band structures shows that Cmcm Si₂₄ is an indirect band gap material, and the value of the band gap is 1.08 eV. In particular, electronic band structures of Cmcm Si₂₄:V exhibit metallic characteristics. Furthermore, the electron localization function indicated strong covalent silicon–silicon bonds in Cmcm Si₂₄ and Cmcm Si₂₄:V. In addition, the effective mass of electrons and holes of Cmcm Si₂₄ is smaller than that of diamond-Si along the $a$ direction, and larger than that along the $b$ and $c$ directions. Finally, the X-ray diffraction patterns for Cmcm Si₂₄ and Cmcm Si₂₄:V are very close after 13.84°C.