Synthesis and Fundamental Studies of Ce_1-x(YLaPrSm)_xO_2-δ as Ionic Conductors

High entropy oxide Y_1/5La_1/5Ce_1/5Pr_1/5Sm_1/5O_2-_δ(F1) with both +3 and +4 preferred cations is considered a promising candidate for oxygen ion conduction since the anion sublattice will contain up to 25% unoccupied sites in a fluorite structure. However, depending on the thermal and kinetic history during its synthesis, F1 may exhibit local chemical ordering and form an unwanted bixbyite-like structure. Therefore, it is crucial to investigate the conditions that lead to the phase transformation and the stabilization of the desired fluorite symmetry in high entropy sesquioxides. In this work, we synthesized a series of bulk F1-derived systems with varying the Ce concentration (Ce_1-x(YLaPrSm)_xO_2-_δ)to investigate the structural dependence on composition. The XRD patterns indicate when the cation mixture is equimolar; the system favors bixbyite structure up to 1500 °C. As the Ce concentration increases beyond 20%, the material transitions into a single-phase fluorite structure without evidence of a bixbyite-like phase. The SEM images reveal that all synthesized Ce_1-x(YLaPrSm)_xO_2-_δ bulk samples have consistent grain size with no significant chemical segregation, as evident from EDS. Moreover, the bandgap and oxidation states of Ce_1-x(YLaPrSm)_xO_2-_δwere investigated by UV-Vis and XPS and supported by first-principles calculations.