Tailoring oxide nanoparticle exsolution in La0.5Ba0.5-yCo1-xFexO3-δ

We show that oxide nanoparticles (NPs) exsolve on La_0.5Ba_0.5-yCo_1-xFe_xO_3-δ (x = 0–1, y = 0 or 0.01) in oxidizing conditions. The phenomenon occurs only in Co-containing materials and depends on pO₂ and pH₂O pressures. Under dry conditions, the smallest NPs average about 30 nm, with 200–300 NPs/µm² at pO₂= 5 × 10⁻⁵ atm. For pO₂= 1 atm, NP size increases to 100–200 nm, and population drops to a few to about 20 NPs/µm² depending on A-site nonstoichiometry and x. In humid conditions, the smallest NPs around 50 nm, with a peak of 100 NPs/µm² exsolve for pO₂= 1. Transmission electron microscopy shows that exsolved NPs in La_0.5Ba_0.5-yCoO_3-δ are Ba-O-rich. We propose defect chemistry models, indicating that exsolution is driven by oxidation reactions forming A-site vacancies, increasing exsolved material with higher pO2. We suggest that adsorbed water under humid conditions blocks nucleation sites, altering observed trends.