Oxygen release behaviour of Ce_(1-x)Zr_xO₂ powders and appearance of Ce_(8-4y)Zr_4yO_(14-δ) solid solution in the ZrO₂-CeO₂-CeO_1.5 system

To clarify the existence of metastable phases in the ZrO₂-CeO₂-CeO_1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce_(1-x)Zr_xO₂) 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 κ phases, (Ce_(1-x)Zr_xO₂) with the composition, x=0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O₂ gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of K phase may be x=0.45∼0.65. A new tetragonal phase t*, (Ce_(1-x)Zr_xO₂), which was attained by cyclic redox process together with annealing in O₂ 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∼0.65. A metastable solid solution expressed by a chemical formula of Ce_(8-4y)Zr_4yO_(14-δ) (y=0∼1) possessing a CaF₂-related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce_(8-4y)Zr_4yO_(14-δ) solid solution was proposed.