Experimental and Theoretical Studies on α‑In2Se3 at High Pressure

α­(R)-In2Se3 has been experimentally and theoretically studied under compression at room temperature by means of X-ray diffraction and Raman scattering measurements as well as by ab initio total-energy and lattice-dynamics calculations. Our study has confirmed the α (R3m) → β′ (C2/m) → β (R3̅m) sequence of pressure-induced phase transitions and has allowed us to understand the mechanism of the monoclinic C2/m to rhombohedral R3̅m phase transition. The monoclinic C2/m phase enhances its symmetry gradually until a complete transformation to the rhombohedral R3̅m structure is attained above 10–12 GPa. The second-order character of this transition is the reason for the discordance in previous measurements. The comparison of Raman measurements and lattice-dynamics calculations has allowed us to tentatively assign most of the Raman-active modes of the three phases. The comparison of experimental results and simulations has helped to distinguish between the different phases of In2Se3 and resolve current controversies.