A high-pressure phase with a non-centrosymmetric crystal structure in the PbSe–PbBr₂ system

We report Pb₃Se₂Br₂, a lead chalcogenide halide that has not been described in the ambient pressure PbSe–PbBr₂ system, synthesized using a high-pressure solid-state method at 4 GPa and 700 °C. Its crystal structure is determined by single crystal X-ray diffraction to be body centered cubic and non-centrosymmetric (Th₃P₄ structure type, space group I4‾3d, #220) with a = 9.3200(4) Å at room temperature and ambient pressure, where it appears to be a metastable phase. The Pb atoms are coordinated by eight anions in a dodecahedron, with Se and Br occupying the same crystallographic site in a disordered 1:1 ratio to yield a normal valence compound. Pb₃Se₂Br₂ is a semiconductor with an indirect band gap of 1.48 eV, determined by diffuse reflectance measurements. The indirect band gap is theoretically supported by density functional theory calculations.