The reduction process from the H-BaFeO2.96 to O-BaFeO2.63 is accompanied by an increase in the lattice parameters. The variation of the lattice parameters (referred to the hexagonal cell) measured at RT shows an increase in the lattice parameters as the oxygen content of the samples decreases, Figure . This variation is related to the oxidation state of iron, from the smaller tetravalent state, present mainly in BaFe 3.92+ O2.96 to the larger trivalent state in BaFe 3.26+ O2.63.