  Silicon telluride (Si2Te3) is a two-dimensional semiconductor with unique
structural properties due to the size contrast between Si and Te atoms. A
recent experiment shows that the material turns metallic under hydrostatic
pressure, while the lattice structure of the metallic phase remains to be
identified. In this paper, we propose two metallic phases, M1 and M2, of Si2Te3
using the evolution algorithm and first-principles density functional theory
(DFT) calculations. Unlike the presence of Si-Si dimers in the semiconducting
(SC) phase, both M1 and M2 phases have individual Si atoms, which play
important roles in the metallicity. Analysis of structural properties,
electronic properties, dynamical as well as thermal stability is performed. The
energies of these new structures are compared with the SC phase under the
subsequent hydrostatic pressure up to 12 GPa. The results show that M1 and M2
phases have lower energies under high pressure, thus elucidating the appearance
of the metallic phase of Si2Te3. In addition, the external pressure causes the
SC phase to have an indirect-direct-indirect bandgap transition. Analysis of
Raman spectra of the SC phase at a different pressure shows the shifting of the
major Raman peaks, and finally disappearing confirms the phase transition. The
results are in good agreement with the experimental observations. The
understanding of the insulator-metal phase transition increases the potential
usefulness of the material system.
