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 (PbMA + MAPb), 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.