Clarifying the Nature of Pressure-Induced Phases of MAPbBr3 by ab initio Molecular Dynamics

Pressure-induced phases of hybrid perovskite MAPbBr3 are investigated at room temperature in a pressure range 0-2.8 GPa by ab initio molecular dynamics. Through appropriately designed MA (MA=methylammonium cation) orientational distribution functions and other order parameters, including a nematic scalar-tensor, we show two transitions at 0.7 and 1.1 GPa involving confinement of MA orientational fluctuations to a crystal plane. The first transition (cubic to cubic) involves dynamic disordering over the plane whereas the second one (cubic to tetragonal) corresponds to a static disordering of MA dipoles along two crystal axes on the same plane. This is similar to isotropic to isotropic and isotropic to oblate transition from the perspective of nematic transitions of liquid crystal. In the latter phase, both local anti-polar and polar domains, consisting of at least two units, are formed. The static disordering of MA dipoles along two crystal axes is stabilized by H-bonding interactions which are also responsible for strong organic-inorganic coupling. The primary order of the pressure-induced phase transitions comes from lattice degrees of freedom most notably octahedral tilting, which is associated with a displacive type transition. These phase transitions are also accompanied by changes in coupled modes between organic and inorganic components such as MA orientations/translations and octahedral tilting/lattice scissoring. Interestingly, the high-pressure phase transition is also driven by suppression of CH3 torsional motion, unlike temperature variation.