Double oxides with the structure of the Ruddlesden–Popper (R-P) layered perovskite A_n+1B_nO_3n+1 attract attention as materials for various electrochemical devices, selective oxygen-permeable ceramic membranes, and catalytic oxidative reactions. In particular, Sr₂TiO₄ layered perovskite is considered a promising catalyst in the oxidative coupling of methane. Our high-resolution transmission electron microscopy (HRTEM) studies of Sr₂TiO₄ samples synthesized using various methods have shown that their structure often contains planar defects disturbing the periodicity of layer alternation. This is due to the crystal-chemical features of the R-P layered perovskite-like oxides whose structure is formed by n consecutive layers of perovskite (ABO₃)_n in alternating with layers of rock-salt type (AO) in various ways along the c crystallographic direction. Planar defects can arise due to a periodicity violation of the layers alternation that also leads to a violation of the synthesized phase stoichiometry. In the present work, a crystallochemical analysis of the possible structure of planar defects is carried out, structures containing defects are modeled, and the effect of such defects on the X-ray diffraction patterns of oxides of the A₂BO₄ type using Sr₂TiO₄ is established as an example. For the calculations, we used the method of constructing probabilistic models of one-dimensionally disordered structures. For the first time, the features of diffraction were established, and an approach was demonstrated for determining the concentration of layer alternation defects applicable to layered perovskite-like oxides of the A₂BO₄ type of any chemical composition. A relation has been established between the concentration of planar defects and the real chemical composition (nonstoichiometry) of the Sr₂TiO₄ phase. The presence of defects leads to the Ti enrichment of particle volume and, consequently, to the enrichment of the surface with Sr. The latter, in turn, according to the data of a number of authors, can serve as an explanation for the catalytic activity of Sr₂TiO₄ in the oxidative coupling of methane.

This study revealed an increased reactivity of centrifugally thermoactivated products of gibbsite toward aqueous solutions of nickel nitrate at room temperature as well as under hydrothermal conditions. X-ray, thermal, microscopy, adsorption and chemical analysis methods were used to investigate and demonstrate the possibility of obtaining highly loaded mixed aluminum–nickel oxide systems, with a nickel content of ca. 33 wt.%, using a hydrochemical treatment at room temperature or a hydrothermal treatment of suspensions of the product of the centrifugal thermal activation of gibbsite in aqueous solutions of nickel nitrate. It was shown that the thermal treatment of xerogels—hydrochemical interaction products—in the range of 350–850 °C led to the formation of NiO phases and highly dispersed solid solutions of nickel based on the NiAl₂O₄ spinel structure, with different ratios and a high specific surface area of 140–200 m²/g. A hydrochemical treatment of suspensions at room temperature ensures that the predominant formation of the NiO phase is distributed over the surface of the alumina matrix after calcination, whereas hydrothermal treatment at 150 °C leads to a deeper interaction of the suspension components at the treatment step, which occurs after the thermal treatment of the formed xerogel in the predominant formation of poorly crystallized NiAl₂O₄ spinel (“protospinel”). The considered method makes it possible to obtain complex aluminum–nickel oxide systems with different phase ratios, decreases the number of initial reagents and synthesis steps, completely excludes waste and diminishes the total amount of nitrates by 75 wt.% compared to the classical nitrate scheme for the coprecipitation of compounds with a similar elemental composition.

Layered perovskite-like Sr₂TiO₄ is a promising catalyst for oxidative coupling of methane (OCM)—a potential direct route to produce C₂ (ethane + ethylene) hydrocarbons, especially, ethylene. In this work, Sr_{2 – х}Ln_xTi_{1 – y}Fe_yO₄ (Ln = La, Pr, х = 0, 0.1; y = 0–0.3) layered perovskites with partially substituted positions of Sr (La, Pr) or Ti (Fe), were synthesized by sol–gel method using polymeric precursors and wetness impregnation followed with calcination at 900–1000°C. The samples were characterized by X‑ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS), adsorption and temperature-programmed hydrogen reduction (H₂-TPR). It was revealed that Sr₂TiO₄ doping with La leads to formation of nanocomposite material containing Sr₂TiO₄, La‑doped SrTiO₃ and La₂O₃ while Sr₂TiO₄, Sr₃Ti₂O₇ and Pr₆O₁₁ are observed in the case of Pr. Fe–doped samples with x = 0.1–0.2 are the single-phase systems. Testing of the obtained doped materials in oxidative coupling of methane has shown the correlation of their catalytic activity with H₂-TPR data and also with the ratio \({\text{O}}_{2}^{{2 - }}\)/O^2– (the surface adsorbed and surface lattice oxygen, correspondingly) and peculiarities of O^2–. The higher СН₄ conversion and C₂ selectivity for the La-doped catalyst is due to appearance of O^2– reduced at medium temperature. The low activity of Pr and Fe doped samples is conditioned by the presence of a large amount of surface adsorbed oxygen \({\text{O}}_{2}^{{2 - }}\) facilitating CO_x formation and decreasing C₂ selectivity.

The effect of strontium deficiency on the microstructure of the Sr₂TiO₄ layered perovskite-like oxide with the crystal structure of the first term of the Ruddlesden–Popper series A_n+1B_nO_3n+1 is studied by powder XRD and high-resolution transmission electron microscopy. It is established that a strontium deficiency of ~5% (Sr_1.9TiO_3.9) can lead to the formation of coherent nano-sized inclusions (lamellas) with the structure of the second term Sr₃Ti₂O₇ of the series in the Sr₂TiO₄ matrix. The simulated XRD patterns show that the presence of such defects explains diffuse scattering effects and the redistribution of intensities of main reflections on the XRD patterns.