We present an 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 presence of any overall polarization arising from the motion of the individual sublattices. Our simulations at room temperature resulted in a cubic Pm3̅ m 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.

The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory...

Pressure-induced phases of MAPbBr 3 were investigated at room temperature in the range of 0−2.8 GPa by ab initio molecular dynamics. Two structural transitions at 0.7 GPa (cubic → cubic) and 1.1 GPa (cubic → tetragonal) involved both the inorganic host (lead bromide) and the organic guest (MA). MA dipoles behave like a liquid crystal undergoing isotropic → isotropic and isotropic → oblate nematic transitions as pressure confines their orientational fluctuations to a crystal plane. Beyond 1.1 GPa, the MA ions lie alternately along two orthogonal directions in the plane forming stacks perpendicular to it. However, the molecular dipoles are statically disordered, leading to stable polar and antipolar MA domains in each stack. H-Bond interactions, which primarily mediate host−guest coupling, facilitate the static disordering of MA dipoles. Interestingly, high pressures suppress CH 3 torsional motion, emphasizing the role of C−H•••Br bonds in the transitions.

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.

Varying the rate at which pressure is applied on a crystal is experimentally known to yield different pressure induced polymorphic structures. Herein, we explore the effect of pressure increase rate on pressure induced polymerization in crystalline acrylamide, using a density functional theory based approach. While quasi-static compression at 0 K stabilizes a 3-dimensional topochemical polymer, Pol-I, at 23 GPa, rapid compression optimizations suggest the presence of multiple polymeric intermediates in the system. Room temperature ab initio molecular dynamics performed with two different compression rates - 0.4 GPa/ps and 2 GPa/ps - revealed very different structural evolution of the system. While both rates ultimately yielded a metastable 1-dimensional polymer at pressures beyond 64 GPa, rapid compression resulted in many disordered polymers at lower pressures with unanticipated linkages. The mechanisms leading to polymerization as well as the structure and electronic properties of the various polymer polymorphs obtained in the two compression routes are described. While large kinetic barriers delay the formation of the thermodynamically favored polymer Pol- I, our simulations suggest a hierarchical route for the pressure induced polymerization of solid acrylamide towards the thermodynamically favorable Pol-I.