## 1 Introduction
The authors synthesized and investigated the structural, magnetic, and magnetocaloric properties of nickel-doped La\({}_{0.67}\)Ba\({}_{0.33}\)Mn\({}_{1-x}\)Ni\({}_{x}\)O\({}_{3}\) (x = 0, 0.025, and 0.075) using the sol-gel method. Nickel doping leads to a systematic decrease in the transition temperature, with a near-room-temperature T\({}_{\rm C}\) (302 K) achieved at the composition x = 0.075, making it a promising candidate for magnetic refrigeration near room temperature [9]. The ceramics La\({}_{0.6}\)Ba\({}_{0.2}\)Sr\({}_{0.2}\)Mn\({}_{1-x}\)Ni\({}_{x}\)O\({}_{3}\) (x = 0.00, 0.05, and 0.1) were synthesized via the sol-gel method in [10]. The Rietveld method identified a rhombohedral phase with space group R c. Based on isothermal magnetization measurements at T\({}_{\rm C}\), the magnetocaloric effects (MCE) were calculated, with maximum entropy change values (\(-\Delta\)SM) of 7.40 J kg\({}^{-1}\)K\({}^{-1}\), 5.6 J kg\({}^{-1}\)K\({}^{-1}\), and 4.48 J kg\({}^{-1}\)K\({}^{-1}\). The La\({}_{0.6}\)Ba\({}_{0.2}\)Sr\({}_{0.2}\)Mn\({}_{1-x}\)Ni\({}_{x}\)O\({}_{3}\) (\(0\leq\) x \(\leq\) 0.1) samples are proposed for use in magnetic refrigeration technology at temperatures above room temperature. In the system of solid solutions Pr\({}_{0.9}\)(Na\({}_{1-x}\)K\({}_{x}\))\({}_{0.1}\)MnO\({}_{3}\), the effect of cation size mismatch at the A-site with constant manganese valence on the structural and magnetic properties of the perovskite-type rare-earth manganite (\(0\leq\) x \(<\) 1) was studied. The in fluence of mixed alkali metal substitution on the structural and magnetic properties of Pr\({}_{0.9}\)(Na\({}_{1-x}\)K\({}_{x}\))\({}_{0.1}\)MnO\({}_{3}\) manganites was analyzed. The results indicate that an increase in potassium (K) content leads to an increase in unit cell volume and a reduction in static Jahn-Teller distortion [11]. The advantages of perovskites include their low cost and non-toxic nature, making compounds with a perovskite structure promising materials. The introduction of a dopant, such as nickel oxide, into the manganite structure opens up possibilities for creating new materials with multifunctional properties, which can serve as a basis for the development of novel technical applications. Nickel manganite rare-earth metals are complex oxide compounds in which rare-earth elements (e.g., Nd) are combined with nickel (Ni) and manganese (Mn) in a perovskite-like crystal structure. These materials have garnered attention due to their unique physical properties, including magnetic, electrical, and catalytic characteristics. The introduction of various cations can distort the structure, affecting the material's physical properties. This approach opens up new opportunities for the application of these materials in advanced technological devices. Moreover, the use of modern synthesis and analysis techniques enables the acquisition of detailed information about the structure and properties of perovskites. Nickel manganites exhibit metallic or semiconducting conductivity depending on their composition and temperature. Their electrical properties can be tuned through cation substitution and oxygen stoichiometry control, making them promising candidates for applications in electronics and spintronics. The study focuses on investigating the physicochemical properties of perovskites doped with alkali metals Na and K, with an emphasis on thermodynamic and electrophysical parameters under phase transition conditions, as well as on examining the processes underlying anomalies in heat capacity and electrical conductivity. Thus, research in the field of nickel manganites of rare-earth metals is actively progressing, and the synthesis of new compounds with various cation combinations opens up opportunities for obtaining materials with tailored properties, which is crucial for their potential application in modern technologies. Due to their diverse properties, nickel manganites of rare-earth metals are considered for use as catalysts, materials for fuel cells, sensors, and memory elements.