High-entropy oxides (HEOs) represent a class of advanced materials derived from the concept of high-entropy alloys, where the stability of a single-phase crystalline structure is enabled by high configurational entropy. By incorporating five or more different metallic cations, it is possible to stabilize single-phase oxide structures, which has opened new opportunities for tailoring the functional properties of materials. Recently, particular attention has been devoted to high-entropy oxides containing rare earth elements, as their specific electronic configurations, ionic radii, and oxidation states allow for additional tuning of structural, optical, and functional properties.
The aim of this master’s thesis was to successfully synthesize a high-entropy oxide based exclusively on rare earth elements. The synthesis was carried out by the gradual incorporation of selected elements, which were chosen according to the criterion of ionic radius size. The high-entropy oxide was synthesized using the citrate–nitrate sol–gel combustion method followed by an additional calcination step. This method was selected due to its simplicity, time efficiency, and cost-effectiveness compared to other synthesis approaches. The process is based on dissolving metal nitrates and complexing metal ions with citric acid, resulting in a homogeneous solution that forms a gel upon drying. Upon ignition, the gel undergoes a self-sustaining combustion reaction, which ensures the initial formation of the material. After the successful synthesis of the high-entropy oxide, the influence of calcination temperature on phase development and microstructure was further investigated.
A high-entropy oxide with the composition (Nd₀,₂Dy₀,₂Pr₀,₂Yb₀,₂Y0.2)ₓOᵧ was successfully synthesized, and the formation of a single-phase material was confirmed, fulfilling the fundamental requirement for exhibiting high-entropy characteristics. It was determined that the material begins to exhibit crystalline properties at 600 °C, while optimal crystallinity is achieved in the temperature range between 1000 and 1100 °C. Specific surface area analysis revealed that the highest surface area was obtained at 600 °C, followed by a gradual decrease with increasing calcination temperature, which can be attributed to crystallite growth and reduced porosity. The results confirm that the selected synthesis method is suitable for the preparation of single-phase rare earth-based high-entropy oxides and enables effective tuning of their structural properties through optimization of the calcination temperature.
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