How binary oxidic overlayers influence the thermochemical stability and surface morphology of (La,Sr)CoO3-δ

• Sr segregation occurs in LSC despite basic or acidic surface modifications. • Surface acidity affects particle formation with acidic gas-phase impurities. • Basic surfaces promote particle formation with acidic ambient gas impurities. • Electrostatic and thermodynamic drivers remain mostly unaffected by surface modifications. Modifying mixed ionic and electronic conductor (MIEC) surfaces has gained attention as a strategy to enhance oxygen exchange reaction kinetics and attenuate surface degradation. This study investigates the high-temperature stability and cation segregation behavior of La 0.6 Sr 0.4 CoO 3-δ (LSC) thin films modified with ∼0.5 nm CaO and SnO 2 overlayers after annealing at 800 °C. Combining Time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), Secondary electron microscopy (SEM)/Energy dispersive X-ray analysis (EDX), and Auger-Meitner electron spectroscopy (AMES), we provide a comprehensive picture of surface and sub-surface changes, segregation, interdiffusion, and secondary phase formation. Our results show that Sr enrichment during high-temperature annealing occurs at the surface regardless of the overlayer. However, significant differences in surface morphology emerge depending on the overlayer. Our results indicate that surface acidity, modulated by the oxide overlayer, is of fundamental importance for the formation of secondary phases and determines the interaction with acidic gas-phase impurities. These findings suggest that surface modifications are not a viable strategy to prevent Sr segregation at high temperatures. However, they can lead to complicated surface dynamics and significantly change the secondary phase formation processes induced by Sr segregation.