Facet-dependent thermal stability of SrTiO₃ surfaces driven by defects, adsorbates, and reconstructions

Oxide surfaces play a crucial role in large-scale applications, including catalysis and electronics. Despite their common use, their surface stability remains a subject of ongoing debate, particularly in computational studies where pristine bulk-terminated models are often used instead of reconstructed, defective, or compositionally modified structures. Here, we use ab initio thermodynamics with explicit phonon-derived vibrational free-energy contributions to evaluate SrTiO₃ surface structures beyond pristine bulk-terminated models, including surface and subsurface defects, double-layer reconstructions, and oxygen coverages across the (001) and (110) facets. At 298 K, the pristine bulk-terminated (001)-AO surface remains stable over most of the allowed chemical-potential window, although Sr-related surface defects become competitive near the TiO₂-rich boundary, while the (001)-BO₂ termination favors defect-containing structures under selected Sr and O chemical-potential conditions. At higher temperatures, these defects become more dominant: at 800 K, the pristine bulk-terminated (001)-BO₂ surface is no longer thermodynamically favored, while at 1300 K, the pristine bulk-terminated (001)-AO surface is stable only within a narrow chemical-potential window. In contrast, the SrO and TiO₂ double-layer reconstructions remain outside the allowed thermodynamic stability window throughout the temperature range examined, indicating that their experimentally observed formation is likely governed by factors beyond the equilibrium thermodynamic framework used here. Moreover, the (110) facet favors defect-containing and oxygen-modified structures over pristine bulk-terminated surfaces across the conditions considered. Together, these findings help reconcile idealized computational models with experimentally observed SrTiO₃ surface behavior, while complementing established experimental and theoretical reconstruction models by showing that once phonon-derived vibrational free energies are explicitly included, defect-containing and oxygen-modified surface structures become thermodynamically preferred under experimentally relevant thermal and chemical conditions.