Post-transition metal and metalloid chalcogenides can have a wide range of unique and valuable properties. ${\mathrm{Bi}}_{0.5}{\mathrm{Sb}}_{1.5}{\mathrm{Te}}_3$ (BST) is part of a family of materials that exhibit multiple functionalities, including thermoelectricity, superconductivity, and electronic topological transitions at high pressure. We have studied the phase transition sequence and equation of state of BST under pressures up to 54 GPa at ambient temperature and 32.5 GPa in the superconducting states using x-ray diffraction. At ambient conditions, BST crystallizes in a hexagonal $R\overline{3}m$ structure and undergoes three structural phase transitions, forming monoclinic C2/m, C2/c, and cubic $\mathit{Im}\overline{3}m$ structures, at 11, 16, and 25 GPa, respectively. Low-temperature diffraction experiments reveal that the superconducting states of BST share the same structures as their room-temperature counterparts. Although the x-ray diffraction clearly indicates that the electronic charge density distribution of the highest-pressure phase has $\mathit{Im}\overline{3}\mathit{m}$ space group symmetry, the symmetry of the underlying atomic lattice depends on the extent of order-disorder of the nuclei. Candidate partially ordered lower symmetry structures of the high-pressure phase are examined in the DFT calculations.

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• Equations of state
• High-pressure studies
• Superconductivity
• Density functional calculations
• X-ray powder diffraction