To determine the oxygen content as well as the complete structural description of the aforementioned 3-fold superstructure phase detected by in situ electron microscopy, several neutron diffraction experiments were attempted. Unfortunately, the elusive experimental conditions to obtain this metastable phase were not found. Nevertheless, considering the 6H packing is kept during the heating and cooling process, an oxygen content between 0 < δ < 0.4 can be suggested (see discussion of tentative structural model in SI), highlighting once more the topotacticity of this system. This fact shows up again the wide range of anionic deficiency that the BaFeO3-δ system can accommodate while preserving the hexagonal structure, a fact which must influence the reversibility of the redox process involved in their catalytic activity.