Deciphering the nature of temperature-induced phases of MAPbBr3 by ab initio molecular dynamics
We present an \textit{ab initio} molecular dynamics study of the temperature-induced phases of methylammonium lead bromide (MAPbBr$_3$). We confirm that the low-temperature phase is not ferroelectric, and rule out the existence of any overall polarization arising from the motion of the individual sub-lattices. Our simulations at room temperature resulted in a cubic \textit{Pm-3m} phase with no discernible local orthorhombic distortions. At low temperatures, such distortions are shown to originate from octahedral scissoring modes, but they vanish at room temperature. The predicted timescales of MA motion agree very well with experimental estimates, establishing dynamic disordering of the molecular dipoles over several orientational minima at room temperature. We also identify the key modes of the inorganic and organic sub-lattices that are coupled at all temperatures mainly through the N-H$\cdots$Br hydrogen-bonds. Estimated lifetimes of the H-bonds correlate well with MA dynamics indicating a strong connection between these two aspects of organic inorganic hybrid perovskites. We also confirm that, in addition to disordering of MA orientations, the transition to the cubic phase is also associated with displacive characteristics arising from both MA as well as Br ions in the lattice.