Deciphering the Nature of Temperature-Induced Phases of MAPbBr₃ by Ab Initio Molecular Dynamics

Abstract

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.

Stabilizing Polar Domains in MAPbBr3 via the Hydrostatic Pressure-Induced Liquid Crystal-like Transition

“…In the low-temperature and ambient-pressure Pnma phase (Figure c), two groups of MAs ( v and w ) are stacked along the a -axis in an AP fashion, yielding a net zero dipole moment . At 0 GPa, a polar minimum [ Pmn2 1 (Figure b)], ∼25 meV/formula unit higher than AP, was also predicted. , Structural optimizations as a function of pressure yield a first-order AP → P transition (Figure and Tables S4 and S5) between 0.7 and 1.0 GPa, suggesting a similar transition to be expected in the MA sublattice at low temperatures. At 300 K, in the ambient-pressure phase, MAs are dynamically disordered, which is due to not only the small enthalpy difference between the AP and P phases but also low MA rotational barriers.…”

“…Figure c shows the MA reorientation time scales extracted by fitting the second-order orientational autocorrelation functions (ACFs), C 2 ( t ) (Figure S10b), to a “wobbling-in-a-cone/reorientational jump” model ,, (see the Supporting Information). The MA remains dynamically disordered from 0.0 to 0.7 GPa, which is evident from the small reorientation time scales (∼2.5 ps).…”

“…20,22,31 These ambiguities in structural transitions are attributed to nonhydrostaticity associated with pressuretransmitting media in experiments. 21,22,31 Previous computational studies revealed the possibility of a polar-ordered MA sublattice at ambient pressure appearing as transient domains at 300 K. 41,54 Pressure could stabilize such domains, thereby inducing polarity in an otherwise nonpolar room-temperature phase of MAPbBr 3 .…”

“…From 1.1 to 2.8 GPa, MA orientations become confined to the b−c plane (see Figure S10a) being restricted to the f f configurations, i.e., ⟨0 ± 10⟩ c and ⟨00 ± 1⟩ c (Figure 3 and Figure S5b), due to large rotational energy barriers (35− 50 meV/formula unit). Furthermore, in the b−c plane, MA axes alternate between the two orthogonal f f orientations, implying long-range in-plane ordering (see the Supporting Figure 4c shows the MA reorientation time scales extracted by fitting the second-order orientational autocorrelation functions 62 (ACFs), C 2 (t) (Figure S10b), to a "wobbling-ina-cone/reorientational jump" model 54,63,64 (see the Supporting Information). The MA remains dynamically disordered from 0.0 to 0.7 GPa, which is evident from the small reorientation time scales (∼2.5 ps).…”

“…Organic–inorganic hybrid perovskites (OIHPs) exhibit a remarkable diversity of structural, optical, and electronic properties for many applications, for example, in solar-cell technology due to their high photoconversion efficiency (PCE) of ≤26%. − The mechanical softness of OIHPs makes them amenable to not only temperature-induced structural transitions − but also pressure-driven transitions to new phases − with changes in hydrogen bonding (H-bonding) and electronic properties. ,, Pressure influences photovoltaic performance by tuning bandgaps, − charge-carrier dynamics, and trap states. , While effects of temperature on (dis)­ordering and coupling between organic–inorganic subsystems have been investigated, ,− studies of their pressure-dependent behavior are rare despite being crucial for understanding structure–property relationships . In this context, ab initio molecular dynamics (AIMD)-based approaches are useful for gaining atomistic insights at high spatial and temporal resolution. ,− …”