Atomic-scale evidence of Sb interdiffusion driving irreversible surface phase transition in MA₃Sb₂I₉ lead-free perovskites

Cooling, rather than annealing, drives surface-to-bulk Sb interdiffusion that irreversibly reconstructs MA₃Sb₂I₉ surfaces. In situ photoemission spectroscopy (PES) and reflection high-energy electron diffraction across the full thermal cycle show that annealing sharpens diffraction streaks and partially depletes surface methylammonium (MA), whereas cooling removes Sb-related spectral features and collapses the streaks. Density-of-states calculations including spin–orbit coupling reproduce the PES evolution, and ab initio molecular dynamics reveal the atomistic mechanism associated with Sb-deficient configurations: local Sb-I coordination collapses and covalent I-I bonds form upon Sb depletion. Treating MA depletion during annealing and Sb interdiffusion during cooling as separate steps explains the observed irreversibility and reconciles mixed reports on A₃B₂X₉ processing. The mechanism provides practical guidance for stabilizing lead-free perovskite surfaces, including passivation before cool-down, halide-rich surface chemistries, and reduced thermal gradients. Beyond MA₃Sb₂I₉, this cooling-aware approach offers transferable rules for processing and reliability in antimony-based, lead-free perovskites.