Rare-earth high-entropy oxides are a new promising class of multifunctional materials characterized by their ability to stabilize complex, multi-cationic compositions into single-phase structures through configurational entropy. This feature enables fine-tuning structural properties such as oxygen vacancies, lattice distortions, and defect chemistry, making them promising for advanced technological applications. While initial research primarily focused on their catalytic performance in energy and environmental applications, recent research demonstrated their potential in optoelectronics, photoluminescent materials, and aerospace technologies. Progress in synthesis techniques has provided control over particle morphology, composition, and defect engineering, enhancing their electronic, thermal, and mechanical properties. Rare-earth high-entropy oxides exhibit tunable bandgaps, exceptional thermal stability, and superior resistance to phase degradation, which positions them as next-generation materials. Despite these advances, challenges remain in scaling up production, optimizing compositions for specific applications, and understanding the fundamental mechanisms governing their multifunctionality. This review provides a comprehensive analysis of the recent developments in rare-earth high-entropy oxides as relatively new and still underrated material of the future.

Double perovskites have been extensively studied in materials chemistry due to their excellent properties and novel features attributed to the coexistence of ferro/ferri/antiferro-magnetic ground state and semiconductor band gap within the same material. Double perovskites with Sr₂NiMO₆ (M = Te, W) structure type have been synthesized using simple, non-toxic and costless aqueous citrate sol-gel route. The reaction yielded phase-pure nanocrystalline powders of two compounds: Sr₂NiWO₆ (SNWO) and Sr₂NiTeO₆ (SNTO). According to the Rietveld refinement of powder X-ray diffraction data at room temperature, Sr₂NiWO₆ is tetragonal (I4/m) and Sr₂NiTeO₆ is monoclinic (C12/m1), with average crystallite sizes of 49 and 77 nm, respectively. Structural studies have been additionally performed by Raman spectroscopy revealing optical phonons typical for vibrations of Te⁶⁺/W⁶⁺O₆ octahedra. Both SNTO and SNWO possess high values of dielectric constants (341 and 308, respectively) with low dielectric loss (0.06 for SNWO) at a frequency of 1 kHz. These values decrease exponentially with the increase of frequency to 1000 kHz, with the dielectric constant being around 260 for both compounds and dielectric loss being 0.01 for SNWO and 0.04 for SNTO. The Nyquist plot for both samples confirms the non-Debye type of relaxation behavior and the dominance of shorter-range movement of charge carriers. Magnetic studies of both compounds revealed antiferromagnetic behavior, with Néel temperature (T_N) being 57 K for SNWO and 35 K for SNTO.

Efficient Lewis-acid-catalyzed direct conversion of aldehydes to 1,2-diketones in the liquid phase was enabled by using newly designed and developed ceria–zirconia-based high-entropy oxides (HEOs) as the actual catalysts. The synergistic effect of various cations incorporated in the same oxide structure (framework) was partially responsible for the efficiency of multicationic materials compared to the corresponding single-cation oxide forms. Furthermore, a clear, linear relationship between the Lewis acidity and the catalytic activity of the HEOs was observed. Due to the developed strategy, exclusively diketone-selective, recyclable, versatile heterogeneous catalytic transformation of aldehydes can be realized under mild reaction conditions.

Four iron-based triple tungstate and tellurate perovskites have been synthesized in the nanocrystalline form using a simple sol–gel citrate route: Sr₃Fe₂WO₉ (SFWO), Ba₃Fe₂WO₉ (BFWO), Sr₃Fe₂TeO₉ (SFTO), and Ba₃Fe₂TeO₉ (BFTO). Strontium-based compounds crystallize in the tetragonal space group I4/m, while barium-based compounds crystallize in the hexagonal space group P6₃/mmc. All compounds possess a ferromagnetic order with high values for room-temperature magnetization. The magnetic state as traced by Mössbauer spectroscopy is heterogeneously related to disorder. The compounds also possess relatively high dielectric constants (20–199.16) measured at 1 MHz and a moderate loss factor. The synthesized triple perovskite compounds were utilized in the fabrication of vertical contact mode triboelectric nanogenerators (TENGs). The working mechanism and the electrical response of the TENGs were extensively studied. The TENG device based on BFTO/Kapton triboelectric layers produces the highest output of 88 V/2.69 μA. For the first time, the surface polarity of the triple perovskite was shown using a Kelvin probe force microscopy technique (KPFM), which elucidates the positive polarity of the triboelectric layer. Furthermore, the TENG device was used to charge several capacitors and to carry out the powering of electronics that make it an excellent choice for various self-powered applications.