Ab initio molecular dynamics study of the nature of pressure-induced orientational phases of methylammonium in MAPbBr3 at room temperature

Pressure-induced orientational phases of methylammonium ions in hybrid perovskite MAPbBr3 are investigated at room-temperature in a pressure range 0-2.8 GPa. Through appropriately designed MA 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 involves dynamic disordering over the plane whereas the second one corresponds to a static disordering of MA dipoles along two crystal axes on the same plane. 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.