The (hcc)2-6H structure for all three compositions was verified by X-ray diffraction and electron microscopy. The X-ray patterns can be indexed on the basis of a hexagonal unit cell with space group P63/mmc; the lattice parameters (listed in Table ) decrease with increasing Fe (IV) content, as expected from the smaller size of Fe (IV), in comparison with Fe (III). direction is shown in Figure . The Ba atomic columns (Z= 56) can be identified as the brightest contrast while Fe atomic columns (Z=26) appear with less bright intensity. The 6H stacking sequence cchcch-can be clearly identified along the c axis (marked by yellow lines). Figure shows the corresponding EELS signal of Fe-L2,3 edge. High energy resolution EELS has demonstrated to be highly sensitive to local variations of the transition metal oxidation state and a clear variation in the shape and the energy position is observed as a function of Fe oxidation state for Fe 2O3 and Ba2Fe (IV) O4 oxides. The The slightly larger width of both peaks, in comparison with that of the reference Fe (IV) signal, is related to the presence of the small, 8%, amount of Fe (III) expected for the BaFeO2.96 composition. Notice that clear differences are observed in both the energy loss and the shape of the Fe-L2,3 signal when a comparison is made with the signal previously reported for BaFeO2.78 (see SI, Fig. ), with a much larger, 44 %, content of Fe (III).