Finite-Temperature Evolution of Frenkel Defects in Hybrid Perovskites: Healing and Lead-Methylammonium Antisite Pairs

Hybrid halide perovskites exhibit remarkable defect tolerance, yet the microscopic origin of this resilience and its limits remain debated. In this work, we employ a combined approach of finite-temperature molecular dynamics and enhanced-sampling metadynamics to investigate the atomistic formation and evolution of Frenkel defects in the prototypical MAPbI₃ lattice. By inducing local perturbations in the stoichiometric crystal, we reconstruct the free-energy profiles and mechanistic pathways for the formation and evolution of defects for all three constituent species. Our results reveal a fundamental difference in the material's defect physics. For the monovalent species (iodine and methylammonium), the soft lattice facilitates rapid self-healing via concerted exchange and direct recombination, effectively suppressing the accumulation of isolated defects. Conversely, for the lead sublattice, the initial perturbation triggers an irreversible structural relaxation into a stable double antisite complex (Pb_MA + MA_Pb), which acts as a deep thermodynamic trap. Large-scale simulations confirm these findings, demonstrating that mobile monovalent defects have a larger interaction range and tend to spontaneously recombine due to short-range instability; while the less mobile lead-based antisites persist as the most energetically favorable separated defect state. These findings provide a mechanistic rationale for the intrinsic self-healing capability of the hybrid framework while identifying the pairs of lead-molecule antisites as the critical bottleneck for long-term electronic stability.