t-Si64 : A Novel Silicon Allotrope.

Utilizing first principle calculations, a novel Si64 silicon allotrope in the I41 /amd space group with tetragonal symmetry (denoted as t-Si64 below) is proposed in this work. In addition, also its structural, anisotropic mechanical, and electronic properties along with its minimum thermal conductivity κmin were predicted. The mechanical and thermodynamic stability of t-Si64 were evaluated by means of elastic constants and phonon spectra. The electronic band structure indicates that t-Si64 is an indirect band gap semiconductor with a band gap: 0.67 eV (primitive cell) compared to a direct band gap of 0.70 eV with respect to a conventional cell. The minimum thermal conductivity of t-Si64 (0.74 W cm-1 K-1 ) is much smaller than that of diamond silicon (1.13 W cm-1  K-1 ). Therefore, Si-Ge alloys in the I41 /amd space group are potential thermoelectric materials.