Cooperative Octahedral Tilt Modes Drive Coexisting Displacive and Order–Disorder Pressure-Induced Phase Transitions in MAPbBr₃

Pressure-induced phases of hybrid perovskite MAPbBr3 were investigated at room temperature in a pressure range 0–2.8 GPa by ab initio molecular dynamics. At ambient pressure, along with the stable global tilted host configurations at the minima of a double-well potential energy surface, low amplitude tilts are also stabilized as shallow single well via MA reorientations. Dynamic disordering across these minima results in a locally cubic host. Pressure induces two structural transitions at 0.7 (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involving both the inorganic (bromide) host and the organic guest (MA). The octahedral tilt modes primarily drive the pressure-induced phase transitions via a mix of disorder-to-order and displacive character in the first and an ordering of layer-wise tilts in the second. The strength of the host/guest coupling increases with pressure, ultimately confining the MA orientational fluctuations and resulting in long-range staggered order in the guest sublattice.