High Pressure Structural Phase Transitions in Silicon Telluride (Si₂Te₃)

Silicon telluride (Si₂Te₃) is a two-dimensional (2D) layered material with potential applications in electronic devices, such as broadband photodetectors and thermoelectrics. Recent experiments based on in-situ Raman spectroscopy at high pressure demonstrated that Si₂Te₃ undergoes a semiconductor-to-metal phase transition at a pressure of 9.5 ± 0.5 GPa. Here, we present a theoretical work that investigates Si₂Te₃ under various pressure conditions and provides a molecular interpretation of these experiments. First, we resolve partial occupations in a Si₂Te₃ supercell with a site permutation search algorithm, to obtain energetically favorable structures at ambient conditions that are suitable as computational models. From first principles calculations with Density Functional Theory (DFT) and spectroscopy simulation with the Density Functional Perturbation Theory (DFPT), we compute the Raman spectra under various pressure conditions which agree well with the experimental Raman spectra. In particular, we observe a phase transition near 9.0 GPa that coincides with the pressure-induced metallic transition in the experimental Raman spectra and an optical color change of the crystal from red to black. At each pressure, we attribute the Raman peaks to vibrational modes in Si₂Te₃, which are characterized by stretching modes of Te layers or stretching and rotation of silicon dumbbells. Using a static compression method within our simulations, we identify a candidate for the high-pressure metallic phase and for the pressure-released semiconductor phase that exhibits a Raman spectrum in excellent agreement with the experiment. Our theoretical methods and results facilitate the investigation of high-pressure phase transitions in Si₂Te₃, offering a technique that applies to other 2D layered structures with partial occupations.