High-Pressure Structure, Equation of State, and superconductivity of Bi0.5Sb1.5Te3: Observation of a Novel Bi-Sb-Te Alloy*

Post-transition metal and metalloid chalcogenides from A₂B₃ (A = Sb, Bi, B = S, Se, Te) is a unique family of materials known to show superconductivity and electronic topological transitions at high pressure. These materials are also of interest due to their thermoelectric properties. Bi_0.5Sb­_1.5Te₃ from the A₂B₃ family is found to show superconductivity with T_c of up to ~9 K under pressure. However, its structural and dynamical properties under pressure have not been fully explored. Here, we investigate the structural and dynamics properties of Bi_0.5Sb­_1.5Te₃ with x-ray diffraction, Raman spectroscopy, and infrared spectroscopy up to 50 GPa. Structural phase transitions observed by X-ray diffraction are consistent with spectroscopic results measured under pressure. An Electronic topological transition inferred at lower pressure can be correlated with Raman measurements. Notably, above ~25 GPa the Bi_0.5Sb­_1.5Te₃ forms a body-centered cubic Bi-Sb-Te alloy with a superconducting T_c near 9 K. Other compounds within the A₂B₃ family are reported to have high-pressure phases with different identified symmetries. These discrepancies are discussed using Raman and infrared measurements.