The parameters of BaFeO2.78 were taken as starting values for refinement. The final results are presented in Figure . Note that a small amount (less than 4 % mass) of Ba2FeO4 is detected by NPD. The oxygen occupancies were refined and the stoichiometry BaFeO2.965(7) was determined for the high-temperature oxidized sample which perfectly matched the chemical analysis result. The small fraction of oxygen vacancies is located in hexagonal layers (O1). The refined structural data are given in To analyze the reduction process, the BaFeO2.96 sample was heated from RT (sample RT1) to 550 °C and cooled to RT under vacuum (sample RT2). Above this temperature, the 6H stacking breaks down due to the random insertion of cubic layers, resulting in an irreversible structural change preventing the analysis of the reduction and oxidation process. Therefore, the highest temperature for the experiment was fixed at 550 ºC. The oxidation process was carried out in air. Under this atmosphere, the RT2 sample was heated again to 450 °C and then cooled to RT (sample RT3). The temperatures where diffraction patterns are collected in both reduction and oxidation processes are gathered in Table . The hexagonal P63/mmc and the orthorhombic Cmcm (maximal non-isomorphous subgroup) cells are related by the transformation matrix (1) giving the following relationships between the hexagonal and orthorhombic unit cell parameters aort = aH; bort = √3aH; cort= cH. The atomic coordinates in this new orthorhombic unit cell are obtained by using the tools provided in the Bilbao Crystallographic Server. (