Therefore, the patterns of the vacuum-reduced samples (named hereinafter O-BaFeO3-δ samples) were fitted to the orthorhombic Cmcm space group. The goodness of fit is indeed greatly improved. The graphic result of the refinement for the RT2-vacuum sample is depicted in Figure and the corresponding structure model is shown in Figure . The in SI. Refinement shows that all orthorhombic samples have a lower oxygen content than the hexagonal ones. The oxygen vacancies are now distributed through both hexagonal (O1) and cubic (O3) layers, although with a higher proportion in the former. The anionic composition from the NDP refinements and calculated Fe average oxidation state for each temperature, are gathered in Table . Ba2FeO4 is added as minority phase (lower ticks). The most reduced sample, attained after cooling down to room temperature in vacuum (RT2-vacuum), O-BaFeO2.63, was taken as the starting sample for the oxidation process.