Pressure effects on the structures and electronic properties of halide perovskite CsPbX₃ (X = I, Br, Cl)

Cesium lead halide perovskites as ideal photovoltaic and optoelectronics materials have attracted more and more attention. Here, we investigated the structure and electronic properties of halide perovskite CsPbX₃ (X = I, Br, Cl) by particle swarm optimization and first principles methods at hydrostatic pressure. For CsPbI₃, the structure phases and corresponding phase transitions of non-perovskite orthorhombic Pnma (non-Pv-Pnma), monoclinic C2/m (C2/m-I), and another different monoclinic phase C2/m (C2/m-II) are found in the pressure range 0-120 GPa. The largest piezochromic effect in the perovskites can be predicted in CsPbI₃ under a pressure below 10 GPa. The band gap is reduced under pressure and the C2/m-II phase becomes a metal under pressures larger than 65 GPa. For CsPbBr₃ and CsPbCl₃, the same phase sequence under pressure, i.e. from perovskite Pnma (Pv-Pnma), to non-Pv-Pnma, C2/m-I, and then to another orthorhombic Cmcm phase, is found. The newly discovered non-Pv-Pnma phase gives an excellent explanation of previous measurements at low pressure for CsPbBr₃ and CsPbCl₃. Furthermore, the calculations of electronic properties show that pressure is an effective means to tune the band gap. Our calculations and results extend the applications in photovoltaics and optoelectronics of halide perovskites.