Pressure-induced isostructural phase transition in methylammonium lead bromide perovskite*

Organic-inorganic hybrid metal-halide perovskites provide a tunable platform for engineering their optoelectronic properties. By combined photoluminescence (PL), synchrotron-based XRD, and Raman scattering studies as a function of pressure from methylammonium lead bromide (MAPbBr₃), we shed light on an isostructural phase transition due to the coupling of the CH₃NH₃ (MA) cation and the PbBr₆ lattice through hydrogen bonding. The PL peak position, intensity and width of the excitonic peak show significant changes at 2 GPa, which corroborate the changes observed in high-pressure Raman scattering studies. The frequencies of the lattice modes and the C–H/N–H bending and stretching modes of the MA cation show anomalous changes at 2 GPa. The suppression of rotational and orientational disorder of the organic moiety is initiated at 2 GPa and the ordering is completed by 3.0 GPa, leading to an order-disorder type cubic II (Im3) to orthorhombic (Pnma) phase transition. Along with the revelation of an isostructural transformation at 2 GPa, this work highlights the impact of molecular vibrations on the electronic properties of MAPbBr₃ under pressure.