Résumé
The thermal oxidation of stoichiometric La2CoO4.00 was investigated using in situ neutron and X-ray powder 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 initiates 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, transitions 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 La2CoO4.263. 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 using Rietveld refinement.