Role of A-site Cation Dynamics in Anisotropic Deformation of Orthorhombic MAPbI₃: A Molecular Dynamics Study

Organic – inorganic halide perovskites such as methylammonium lead iodide (MAPbI₃) exhibit exceptional optoelectronic properties but suffer from mechanical fragility that compromises long-term device integrity. In particular, their soft hybrid lattice gives rise to complex, direction-dependent mechanical responses that remain poorly understood. Here, we employ classical molecular dynamics simulations to investigate the anisotropic mechanical behavior of orthorhombic MAPbI₃ under uniaxial tension and compression. The stress–strain response reveals pronounced elastic anisotropy, with substantially lower stiffness and failure strength along the [001] direction compared to the [100] direction. This anisotropy originates from directional differences in PbI octahedral connectivity and weakly bonded organic–inorganic stacking along [001]. Our analysis reveals that uniaxial loading, regardless of direction, suppresses methylammonium (MA) cation rotation dynamics, culminating in molecular locking, with the notable exception of tensile loading along [001]. Molecular locking decouples the organic and inorganic sublattices, driving structural amorphization and ultimately brittle-like mechanical instability. In contrast, under tension along [001], MA molecules retain the freedom to rotate out of the xy-plane; this preserved cation dynamics maintains hydrogen bonding with the inorganic PbI₆ framework and gives rise to ductile-like behavior. Additionally, MAPbI₃ exhibits pronounced tension–compression asymmetry, wherein localized amorphization under tensile loading results in reduced strength and failure strain relative to compression. These mechanistic insights into strain-dependent cation dynamics and structural decoupling provide a foundation for strain-engineering strategies aimed at enhancing the mechanical robustness of hybrid halide perovskites.