This paper reports synchrotron X‐ray powder diffraction and high‐resolution electron microscopy studies of the structural arrangement in γ‐Fe2O3. Powder diffraction data as well as high‐resolution electron microscopy show that the basic structure of γ‐ferric oxide possesses cubic symmetry (space group P4332), whereas the ordered distribution of the cation vacancies on the octahedral positions results in the formation of the tetragonal superlattice (space group P41212). The character of the superstructure orientation with respect to the developed planes of the γ‐Fe2O3 microcrystals obtained by different preparative techniques is also discussed.

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A series of Mn−Co mixed oxides with a gradual variation of the Mn/Co molar ratio were prepared by coprecipitation of cobalt and manganese nitrates. The structure, chemistry, and reducibility of the oxides were studied by X-ray diffraction (XRD), X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). It was found that at concentrations of Mn below 37 atom %, a solid solution with a cubic spinel structure is formed. At concentrations above 63 atom %, a solid solution is formed on the basis of a tetragonal spinel, while at concentrations in a range of 37−63 atom %, a two-phase system, which contains tetragonal and cubic oxides, is formed. To elucidate the reduction route of mixed oxides, two approaches were used. The first was based on a gradual change in the chemical composition of Mn−Co oxides, illustrating slow changes in the TPR profiles. The second approach consisted in a combination of in situ XRD and pseudo-in situ XPS techniques, which made it possible to directly determine the structure and chemistry of the oxides under reductive conditions. It was shown that the reduction of Mn−Co mixed oxides proceeds via two stages. During the first stage, (Mn, Co) 3 O 4 is reduced to (Mn, Co)O. During the second stage, the solid solution (Mn, Co)O is transformed into metallic cobalt and MnO. The introduction of manganese cations into the structure of cobalt oxide leads to a decrease in the rate of both reduction stages. However, the influence of additional cations on the second reduction stage is more noticeable. This is due to crystallographic peculiarities of the compounds: the conversion from the initial oxide (Mn, Co) 3 O 4 into the intermediate oxide (Mn, Co)O requires only a small displacement of cations, whereas the formation of metallic cobalt from (Mn, Co)O requires a rearrangement of the entire structure.

Despite the fact that metastable aluminum oxides are actively used in industry, there is a discrepancy in the literature regarding their crystal structure. All this leads to difficulties in data interpretation and, as a consequence, classification problems. This work is aimed at solving these tasks. The main features of powder X-ray diffraction of typical samples of three Al2O3 polymorphs (γ-, χ-, η-) are analyzed. Specifics and fundamental differences in X-ray scattering and their relationship with the structural organization at the nanostructure level are clearly shown. The work demonstrates the possibilities of analyzing experimental powder X-ray diffraction data using a modern approach based on the Debye Scattering Equation for studying the organization of such complex systems.