Pressure-induced metallization and tetravalent antimony formation in the mixed-valent defect perovskite Cs₂SbBr₆

We report high-pressure investigations of the mixed-valent perovskite Cs2SbBr6, in which Sb3+ and Sb5+ cations coexist, and two types of distorted SbBr6 octahedra are arranged without connectivity, forming a zero-dimensional structure. The Cs2SbBr6 halide undergoes a concurrent pressure-induced tetragonal-to-cubic phase transition and metallization at approximately 3.60 GPa, maintaining its crystalline form up to the maximum studied pressure of 48.20 GPa. Simultaneously, the mixed-valent Sb3+/Sb5+ transforms into single-valent Sb4+, leading to an increase in SbBr6 symmetry from D4h to Oh. Electrical resistance measurements show a large reduction in resistance by almost five orders of magnitude during the semiconductor-to-metal transformation, which then increases with further compression. No signature of superconductivity was observed down to 2.8 K. Density functional theory (DFT) calculations indicate that metallization emerges in the high-pressure cubic phase and further suggest that a reduction in the density of states near the Fermi level is likely responsible for the increased resistivity at higher pressures. This study reveals the role of Sb valence states in governing phase stability and metallization in mixed-valent halides, providing insights into the design of related inorganic halide perovskites with intriguing correlated electronic states.