This study explores the influence of two different synthesis methods on the sintering behavior of three novel high-entropy oxides possibly suitable for thermal barrier applications: (Ce_0.2Zr_0.2Yb_0.2Er_0.2Nd_0.2)O_2-δ, (Ce_0.2Zr_0.2Yb_0.2Er_0.2La_0.2)O_2-δ, and (Ce_0.2Nd_0.2Yb_0.2Er_0.2La_0.2)₂O_3+δ. Rare-Earth-based High-Entropy Oxides (RE-HEOs), recently known for their exceptional thermal stability and compositional flexibility, have gained increasing attention as potential candidates for many advanced technological applications. Thus, our current work focuses on the specific effects of synthesis techniques, namely co-precipitation and hydrothermal treatment, on the entropy-driven stabilization, microstructure, electrochemical properties, and sintering behavior of three novel RE-HEOs. The obtained results reveal significant differences in terms of densification yield and of the obtaining of the designed entropy-stabilized single phase depending on the adopted synthesis route, underscoring the critical role of synthesis in optimizing RE-HEOs for near-future technological applications.

The reduction of nitrogen oxides (NO_x), critical pollutants from stationary to mobile sources, mainly relies on the selective catalytic reduction (NH₃-SCR) method, employing ammonia to reduce NO_x into nitrogen and water. However, conventional catalysts, while effective, pose both environmental and operational challenges. This study investigates ceria-zirconia-supported molybdenum-based catalysts, exploring the effects of zirconium doping and different catalyst synthesis techniques, i.e., co-precipitation and impregnation. The catalytic performance of the differently prepared samples was significantly influenced by the molybdenum incorporation method and the zirconium content within the ceria-zirconia support. Co-precipitation at higher temperatures resulted in catalysts with better structural attributes but slightly lower catalytic activity compared to those prepared via impregnation. Optimal NO_x reduction (close to 100%) was observed at a 15 mol% zirconium doping level when using the impregnation method.

Rare-earth hydroxycarbonates and oxycarbonates are attractive functional materials because their crystal chemistry and optical response can be tailored through controlled cation substitution. In this work, Eu-doped lanthanum hydroxycarbonates with nominal europium contents of 1, 3, and 5 mol% were synthesized by combining co-precipitation and hydrothermal treatment at 140 °C for 24 h and subsequently calcined at 500 °C for 0.5 h to obtain the corresponding oxycarbonates. X-ray diffraction showed that the as-synthesized powders consist of single-phase hexagonal LaCO₃OH, while the calcined products are single-phase La₂O₂CO₃. In both structural families, systematic peak shifts with increasing Eu content indicated the formation of homogeneous substitutional solid solutions. Thermal analysis revealed a clear two-step decomposition pathway for the hydroxycarbonate precursors, with endothermic events at about 530 and 850 °C, consistent with the sequential transformation from hydroxycarbonate to oxycarbonate and, finally, to oxide. UV-Vis absorption measurements highlighted a dopant-dependent shift in the absorption edge in both hydroxycarbonate and oxycarbonate systems. Kubelka–Munk analysis showed that the estimated band-gap energy increases with Eu content, from 4.9 to 5.4 eV for LaCO₃OH-based samples and from 4.7 to 5.1 eV for La₂O₂CO₃-based samples. These results demonstrate that europium incorporation is an effective strategy for tuning the structural evolution and optical properties of lanthanum carbonate-derived materials, thus supporting their potential use in UV-responsive rare-earth-based functional systems.

This work presents the design and synthesis of novel high-entropy perovskite oxides (HEPOs) derived from BaCeO₃, formulated using the cluster-plus-glue atom model. Particularly, through a carbonate-based co-precipitation technique, we synthesized three novel high-entropy perovskite oxides (HEPOs) derived from barium cerate by substituting cerium with different combinations of five different elements (Ce, Zr, Yb, Sm, La, Gd, Nd) in equal molar ratios, i.e., Ba(Ce_0.2Zr_0.2Yb_0.2La_0.2Sm_0.2)O_2.7, Ba(Ce_0.2Sm_0.2Yb_0.2Nd_0.2Gd_0.2)O_2.6, and Ba(Ce_0.2Zr_0.2Nd_0.2La_0.2Sm_0.2)O_2.7. Upon calcination of the as-synthesized samples at different temperatures and subsequent quenching, the formation of an entropy-stabilized single phase was analyzed and assessed. To rationalize the observed differences in phase evolution, a novel set of empirical descriptors, including configurational entropy, Goldschmidt tolerance factor, and B-site size mismatch, was proposed and discussed. With the aim of studying the sinterability of the single-phase samples, the calcination treatment was optimized by reducing its temperature and duration (i.e., 1300 °C for 6 h) so that subsequent densification higher than 95% was achieved by sintering at 1500 °C for 6 h.