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

Abstract

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.