On the effect of kinetics on the formation of fluorite/bixbyite-structured Entropy-Stabilized Oxides

The exploration of novel entropy-stabilized oxides (ESOs), particularly those with fluorite or bixbyite structures, has recently attracted considerable attention due to their potential technological applications, including thermal barrier coatings and catalysts/photocatalysts. This study seeks to identify optimal synthesis pathways that lower kinetic barriers and expedite the formation of the entropy-stabilized single phase, thereby contributing to more efficient fabrication processes for these complex materials, which hold promise for a wide range of technological uses. In this context, two distinct rare-earth-based systems, i.e. (Ce0.2Zr0.2Yb0.2Er0.2La0.2)O1.7 and (Ce0.2Nd0.2Er0.2Yb0.2La0.2)O1.6, were synthesized via co-precipitation (using ammonia and ammonium carbonate as precipitating agents) and through the conventional solid-state method. A detailed structural analysis revealed that precursor reactivity strongly influences the transition to a single-phase fluorite-like or bixbyite-like high-entropy oxide (HEO). Specifically, precursors derived from ammonium carbonate displayed higher reactivity, promoting phase formation at lower temperatures, while ammonia-based precursors required higher transition temperatures due to agglomeration effects. Conversely, the solid-state synthesis route exhibited reduced reactivity, thereby delaying (or even inhibiting) phase transitions in the more complex bixbyite-like system. Thus, our findings highlight the critical role of kinetic barriers in forming entropy-stabilized fluorite-like and/or bixbyite-like structures and underscore the importance of the chosen synthesis cycle in optimizing HEO fabrication for future technological applications.