Unraveling 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. We find two structural transitions at 0.7 and 1.1 GPa involving confinement of MA orientational fluctuations to a crystal plane - one (cubic to cubic) involving dynamic disordering over the plane and another (cubic to tetragonal) corresponding 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 two transitions are primarily driven by octahedral tilting modes of the host lattice involving a displacive character in the first and an ordering of layer-wise tilts in the second transition. Coupling between the MA (guest) orientations/translations and octahedral tilting/lattice scissoring in the inorganic host are also altered along the transitions. H-bonding interactions, which primarily mediate host/guest coupling, facilitate the static disordering of MA dipoles along two crystal axes. Unlike temperature-driven transitions in the system, high pressures suppress CH3 torsional motion emphasizing the role of C-H· · · Br bonds in driving the transitions.