A Novel Silicon Allotrope in the Monoclinic Phase.

This paper describes a new silicon allotrope in the 2/ space group found by first-principles calculations using the Cambridge Serial Total Energy Package (CASTEP) plane-wave code. The examined 2/-Si belongs to the monoclinic crystal system. 2/-Si is an indirect band-gap semiconductor with a band gap of 1.51 eV, as determined using the HSE06 hybrid functional. The elastic constants, phonon spectra and enthalpy indicate that 2/-Si is mechanically, dynamically, and thermodynamically stable. 2/-Si is a low-density (2.19 g/cm³) silicon allotrope. The value of / is less than 1.75, which indicates that the new allotrope is brittle. It is shown that the difference in the elastic anisotropy along different orientations is greater than that in other phases. Finally, to understand the thermodynamic properties of 2/-Si, the thermal expansion coefficient , the Debye temperature , and the heat capacities and are also investigated in detail.