Insights Into Low‐Temperature Cation Ordering in Fe‐Added Ce–Zr‐Based Oxides

CeO₂-ZrO₂ (CZ) solid solutions are widely utilized to control the oxygen partial pressure of automobile exhaust purification systems owing to their high oxygen storage capacity (OSC) related to the valence change of Ce ions upon reduction. Among various CZs, cation-ordered κ-Ce₂Zr₂O₈ shows the highest OSC; however, the ordering requires high-temperature reduction above 1200 °C, causing grain growth and potentially compromising the OSC. Recently, it has been reported that adding a small amount of Fe₂O₃ to CZ (Zr/Ce = 1) lowers the ordering temperature to 800 °C. In this study, Zr-rich CZ, known for its excellent heat resistance and widespread applications, is cation-ordered at low temperatures by the addition of Fe₂O₃. Using high-temperature in situ XRD, the low-temperature ordering behavior of Fe₂O₃-added Zr-rich CZ is observed under oxygen partial pressure during reduction. A weakly reducing atmosphere promotes CZ ordering because Fe₂O₃ remains an ionic Fe that can be dissolved in CZ to facilitate cation migration. In contrast, a strongly reducing atmosphere converts Fe₂O₃ to metallic Fe, which is unfavorable for CZ ordering. The study suggests that the reduction atmosphere has a significant impact on the dissolution of transition metal oxides and cation ordering of ceramics.