Carrier-Lattice Coupling Drives Oxidative Degradation in Mixed Tin-Lead Halide Perovskites: Machine Learning Combined with Nonadiabatic Molecular Dynamics.

Mixed Sn-Pb halide perovskites exhibit excellent optoelectronic properties but suffer rapid degradation due to the facile oxidation of Sn2+ to Sn4+. Here, we establish a multiscale framework to uncover how cation ordering, surface defect chemistry, hole localization, and carrier dynamics collectively drive the initiation of Sn oxidation in FAPb0.5Sn0.5I3 (FA = formamidinium). Graph neural network screening identifies layered Sn/Pb enrichment as a recurring low-energy motif, which naturally yields an Sn-rich, Sn-I-terminated surface. Static calculations reveal contrasting effects of the two surface defects: the Sn vacancy (VSn) creates an oxidation-prone local environment that stabilizes Sn-centered hole localization and lattice distortion, whereas atop iodine (Iatop) promotes hole delocalization, reinforces the surface Sn-I framework, and suppresses the formation of Sn(IV)-like oxidation precursors. Nonadiabatic molecular dynamics further reveal a hundred-picosecond window, defined by ultrafast hole extraction toward the surface and much slower nonradiative recombination, in which accumulated photogenerated holes can drive oxidation-related lattice reorganization. Ab initio molecular dynamics simulations with two excess holes show that the pristine surface remains relatively robust, whereas VSn strongly stabilizes localized holes and triggers pronounced local distortion together with SnI4-like extrusion and defect growth. These findings reveal that Sn oxidation is governed by carrier-lattice coupling through its control over the kinetic accessibility of Sn-centered hole localization, and they identify the suppression of VSn defects and the stabilization of I-rich surface terminations as key design principles for stable Sn-Pb perovskites.