K _x Ln _{5− x} Mo ₃ O _{16− y} F _y (Ln = La, Pr, Nd) compositions were first synthesized. The conductive properties have been widely studied using a combination of theoretical and experimental approaches. The total conductivity reaches ∼10 ⁻² S cm ⁻¹ at 800 °C.

The fluorine-containing fluorite-like compounds of composition NaLn₄Mo₃O₁₅F (Ln = La, Pr, Nd) are produced using the method of solid-phase synthesis in air. Their thermomechanical and conducting properties, as well as hygroscopicity, are studied. It is confirmed that the synthesized specimens are isostructural to the cubic compounds Ln₅Mo₃O₁₆ with the fluorite-like structure. It is shown that in the temperature range of 20–600°C, the specimens of NaLn₄Mo₃O₁₅F (Ln = La, Pr, Nd) expand linearly, and their thermal expansion coefficients (13–14) × 10^–6 K^–1 are close to the coefficients of conventional SOFC electrolytes, for example, YSZ. Using the thermal gravimetric analysis, it is shown that the weight loss of the studied specimens in the range from 30 to 700°C is caused by their hygroscopicity. The electrophysical properties of the compounds are studied using the method of impedance spectroscopy in the humid atmosphere, and the proton component of conductivity is revealed.

The mechanism of phase formation from the initial and the mechanically activated (m/a) mixture of Sm₂O₃+MoO₃ oxides is studied by differential scanning calorimetry (DSC) in an oxygen atmosphere. It is shown that different mechanisms of samarium oxymolybdate synthesis are realized in these two cases. As a result of the mechanochemical action at room temperature, a nanosized mixture of Sm₂(MoO₄)₃ and Sm₂O₃ is formed. Upon heating, Sm₂(MoO₄)₃ is crystallized at the first stage and its interaction with Sm₂O₃ in the second stage at 900°C leads to the synthesis of oxymolybdate Sm₂MoO₆ with a scheelite structure, and this structure type is stable up to 1400°C. The kinetic experiment in a DSC cell shows only an apparent similarity of the phase formation mechanism with a decrease of the main exoeffects by 70°C for a m/a mixture of oxides. At the same time, the study of the mechanism of phase formation by isothermal exposure at different temperatures reveals the main advantages of ceramic synthesis from an activated oxide mixture. The total conductivity of Sm₂MoO₆ with a scheelite structure, which turned out to be of the p-type (1 × 10⁻⁶ S/cm at 600°C), is studied.

Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) solid electrolytes have been investigated as potential electrolyte materials for solid oxygen fuel cells (SOFCs), operating in the medium temperature range (600–700 °C). The design of new oxygen-conducting materials is of importance for their possible utilization in the solid oxide fuel cells. The oxygen–ion conductivity of the Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0, 0.1, 0.18, 0.28, 0.74) “stuffed” pyrochlores ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) in dry and wet air. The synthesis of precursors via co-precipitation and the precipitate decomposition temperature have been shown to be of key importance for obtaining dense and highly conductive ceramics. At ~770 °C, the highest total conductivity, ~3.16 × 10⁻³ S/cm, is offered by Tm₂Ti₂O₇. The conductivity of the fluorite-like solid solution Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) is an order of magnitude lower. However, for the first time a proton contribution of ~5 × 10⁻⁵ S/cm at 600 °C has been found in Tm₂(Ti_2−xTm_x)O_7−x/2 (x = 0.74) fluorite. Until now, compositions with proton conductivity were not known for the intermediate and heavy rare earth titanates Ln₂(Ti_2−xLn_x)O_7−x/2 (Ln = Ho − Lu) systems. The use of X-ray diffraction (structural analysis with Rietveld refinement), optical spectroscopy and dielectric permittivity data allowed us to follow structural disordering in the solid solution series with increasing thulium oxide content. High and low cooling rates have been shown to have different effects on the properties of the ceramics. Slow cooling initiates’ growth of fluorite nanodomains in a pyrochlore matrix. The fabrication of such nanostructured dense composites is a promising direction in the synthesis of highly conductive solid electrolytes for SOFCs. We assume that high-temperature firing of nanophase precursors helps to obtain lightly doped “stuffed” pyrochlores, which also provide the high oxygen–ion conductivity.

A modified version of the use of the sol-gel process at the initial stage of the synthesis of a high-temperature superconductor Y_1–xFe_xBa₂Cu₃O_y with low Fe doping is proposed. The properties of the samples obtained by sol-gel and solid-state synthesis are compared. It is shown that a more uniform distribution of the dopant throughout the volume of the sol-gel samples makes it possible to obtain materials with improved microstructural and performance characteristics.