Exploring Librational Pathways with on-the-Fly Machine-Learning Force Fields: Methylammonium Molecules in MAPbX3 (X = I, Br, Cl) Perovskites

Atomic Structure and Dynamics of Organic-Inorganic Hybrid Perovskite Formamidinium Lead Iodide

The atomic dynamic behaviors of formamidinium lead iodide [HC(NH2)2PbI3] are critical for understanding and improving photovoltaic performances. However, they remain unclear. Here, we investigate the structural phase transitions and the reorientation dynamics of the formamidinium cation [HC(NH2)2+, FA+] of FAPbI3 using neutron scattering techniques. Two structural phase transitions occur with decreasing temperature, from cubic to tetragonal phase at 285 K and then to another tetragonal at 140 K, accompanied by gradually frozen reorientation of FA cations. The nearly isotropic reorientation in the cubic phase is suppressed to reorientation motions involving a two-fold (C2) rotation along the N···N axis and a four-fold (C4) rotation along the C-H axis in the tetragonal phase, and eventually to local disordered motion as a partial C4 along the C-H axis in another tetragonal phase, thereby indicating an intimate interplay between lattice and orientation degrees of freedom in the hybrid perovskite materials. The present complete atomic structure and dynamics provide a solid standing point to understand and then improve photovoltaic properties of organic-inorganic hybrid perovskites in the future.

Structural Dynamics Descriptors for Metal Halide Perovskites

Metal halide perovskites have shown extraordinary performance in solar energy conversion technologies. They have been classified as "soft semiconductors" due to their flexible corner-sharing octahedral networks and polymorphous nature. Understanding the local and average structures continues to be challenging for both modeling and experiments. Here, we report the quantitative analysis of structural dynamics in time and space from molecular dynamics simulations of perovskite crystals. The compact descriptors provided cover a wide variety of structural properties, including octahedral tilting and distortion, local lattice parameters, molecular orientations, as well as their spatial correlation. To validate our methods, we have trained a machine learning force field (MLFF) for methylammonium lead bromide (CH3NH3PbBr3) using an on-the-fly training approach with Gaussian process regression. The known stable phases are reproduced, and we find an additional symmetry-breaking effect in the cubic and tetragonal phases close to the phase-transition temperature. To test the implementation for large trajectories, we also apply it to 69,120 atom simulations for CsPbI3 based on an MLFF developed using the atomic cluster expansion formalism. The structural dynamics descriptors and Python toolkit are general to perovskites and readily transferable to more complex compositions.

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

Pressure-induced phases of MAPbBr₃ 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 CH₃ torsional motion, emphasizing the role of C-H···Br bonds in the transitions.

Disorder to order: how halide mixing in MAPbI3-xBrx perovskites restricts MA dynamics

Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI_3-xBr_x perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). ²⁰⁷Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI_3-xBr_x samples are cubic. The experimental ¹⁴N spectra and ¹H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb₈I_12-nBr_n "cages". Based on the experimental and simulated results, we develop a phenomenological model that correlates the ¹H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb-X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.