Variations in the Extent of Pyrochlore-Type Cation Ordering in Ce₂Zr₂O₈: A t‘−κ Pathway to Low-Temperature Reduction

Abstract

A Ce2Zr2O8 mixed oxide was prepared, and the influence of redox treatments on its reduction behavior was systematically analyzed. The temperature of reduction was found to change on application of redox cycles under severe conditions, with the final appearance of temperature-programmed reduction profiles dictated by the exact conditions employed in the redox cycle (temperature of treatment and concentration of H2). In addition, the conditions necessary in the reducing and oxidizing parts of the redox cycle showed interdependence. The reduction was found to vary from a single high-temperature peak to a single low-temperature peak, via double-peak profiles with various relative intensities. Structural analysis of the sample after application of analogous treatments indicated that the reduction behavior follows a pathway, with t‘ and κ phases at the two extremes (high- and low-temperature single-peak profiles). The latter phase contains pyrochlore-type cation ordering. However, bulk ordering is not necessary to promote low-temperature reduction. The evidence suggests that the two-peak profiles are observed when cation ordering is very limited, and a single low-temperature peak is observed when domains of the κ phase coexist with larger amounts of the t‘ phase. Thus, even the presence of partial ordering at the surface is sufficient to produce an effect.