Reaction Intermediates and Oxygen Ordering Explored by In Situ Neutron Powder Diffraction during Thermal Oxidation of La ₂ CoO _4.00

The thermal oxidation of stoichiometric La₂CoO_4.00 was investigated using in situ neutron and powder X-ray diffraction in air, complemented by thermogravimetric analysis (TGA). The process involves the topotactic insertion of extra oxygen atoms into interstitial lattice sites, forming La₂CoO₄₊_δ. Oxygen uptake is initiated above 320 K, leading to a sequence of phase transitions as a function of temperature (T) and oxygen excess (δ). The structural evolution starts from an orthorhombic phase at δ = 0, with a transition to a tetragonal phase for 0.05 ≤ δ ≤ 0.17, and proceeds through two distinct orthorhombic phases, both exhibiting long-range oxygen ordering above 525 K. The maximum oxygen concentration, observed above 680 K, corresponds to La₂CoO_4.263. The resulting phase diagram reveals rapid oxygen diffusion and ordering kinetics, accompanied by subtle structural modifications as a function of δ. Notably, the long-range oxygen order remains stable over a wide temperature range. Each phase was structurally characterized by using Rietveld refinement.