posted on 2022-11-28, 16:06authored bySayan Maity, Suraj Verma, Lavanya M. Ramaniah, Varadharajan Srinivasan
We present an ab initio molecular dynamics
study
of the temperature-induced phases of methylammonium lead bromide (MAPbBr3). We confirm that the low-temperature phase is not ferroelectric
and rule out the presence of any overall polarization arising from
the motion of the individual sublattices. Our simulations at room
temperature resulted in a cubic Pm3̅m 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 sublattices that are coupled
at all temperatures, mainly through the N–H···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 the disordering of MA orientations, the transition
to the cubic phase is also associated with displacive characteristics
arising from both MA and Br ions in the lattice.