Redox behavior and phase stability in binary, ternary, and high-entropy oxide solid solutions

This review of redox behavior in binary, ternary and high-entropy oxide (HEO) solid solutions demonstrates that the oxidation state of multivalent cations is closely related to the inherent properties of the elements, which are not strongly affected by crystal structure except in dual-valent pure oxides like CeOx. Oxidation states of bivalent cations in ternary (and likely in high-entropy) oxides vary linearly or follow a smooth curve versus temperature at a given oxygen partial pressure, showing a greater reduction as temperature increases. While defect clustering or nanodomains of order often occur in solid solutions, diffusion rates are so low that ordered intermediate phases do not form at temperatures where they are stable. This review is inclusive regarding the behavior of lanthanide and actinide containing binary/ternary oxides but is limited regarding HEO systems, as only some studies incorporate Ce, Pr, Tb, or other multivalent cations as focused on here, and none have been reported that contain actinide elements except various studies have been reported in nuclear waste management involving development of very complex oxides that could be considered HEO ceramics though not identified as such, nor studied in that way.