This process was carried out by heating the sample to 450 °C and then cooling to RT in air (RT3-air). In this temperature range, the redox process is reversible so that, during air cooling, the hexagonal symmetry is recovered from 300 °C, attaining a hexagonal symmetry phase with H-BaFeO2.86 composition at RT. The refined crystallographic parameters and bond distance values for the RT3-air sample are given in Tables and, respectively. Refinement results from ND data collected at intermediate temperatures during both heating and cooling processes are gathered in SI (Tables and) we were able to find the aforementioned splitting maxima associated with the orthorhombic distortion. A better fit with orthorhombic symmetry is provided in the supplementary information (Fig. and Table ), which widens the stability range of orthorhombic symmetry to near BaFeO2.63-2.78.