Oxygen release behaviour of Ce(1-x)ZrxO2 powders and appearance of Ce(8-4y)Zr4yO(14-δ) solid solution in the ZrO2-CeO2-CeO1.5 system

To clarify the existence of metastable phases in the ZrO2-CeO2-CeO1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t' and t" (Ce(1-x)ZrxO2) with various compositions, x, in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t' and t" phases was very complicated. The single kappa phases, (Ce(1-x)ZrxO2) with the composition, x=0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in 0, gas at 873 K, enhibited a sharp oxygen release at the lowest temperature; the composition range of kappa phase may be x=0.45 similar to 0.65. A new tetragonal phase t* (Ce(1-x)ZrxO2), which was attained by cyclic redox process together with annealing in O-2 gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x=0.20 similar to 0.65. A metastable solid solution expressed by a chemical formula of Ce(8-4y)Zr4yO(14-delta) (y=0 similar to 1) possessing a CaF2-related structure appeared on deoxidation of the t*phase. A ternary phase diagram containing the t* and Ce(8-4y)Zr4yO(14-delta) solid solution was proposed. (C) 1998 Elsevier Science S.A.