Vibrational Spectroscopic and X-Ray Diffraction Studies of Cerium Zirconium Oxides with Ce/Zr Composition Ratio=1 Prepared by Reduction and Successive Oxidation of t′-(Ce_0.5Zr_0.5)O₂ Phase

Cerium zirconium oxides with the composition CeZrO₄ were prepared by oxidizing in O₂ at 873 K precursors with the composition CeZrO_3.5+δ (δ<0.3). The precursors were prepared by reducing the t′-(Ce_0.5Zr_0.5)O₂ phase at 873≤T_red.≤1573 K. The CeZrO₄ compounds together with the precursors were characterized by powder X-ray diffraction and IR and Raman spectroscopies. In the case of 1323≤T_red.≤1573 K, the precursors were identified as having the pyrochlore-type Ce₂Zr₂O_7+2δ phase with δ<0.06. When the reduction temperature was decreased, the ordering level between Ce and Zr ions was decreased in the pyrochlore-type phase. The CeZrO₄ compounds obtained by oxidizing the pyrochlore-type precursors were identified as the κ-CeZrO₄ phase in which Ce and Zr ions are in an ordered arrangement similar to that of the pyrochlore-type structure. The space group of the κ phase was not the same as that of the pyrochlore-type structure. In the case of T_red.≤1123 K, cubic CaF₂-related-type (Ce_0.5Zr_0.5)O_(3.5+δ)/2 phases with 0.1<δ<0.3 were obtained. When the precursor obtained at T_red.=1123 K was oxidized, a tetragonal phase appeared, but its Raman spectrum was different from that seen in the t′ phase. This phase was identified to be a novel t′_meta-(Ce_0.5Zr_0.5)O₂ phase, because it transformed into the t′ phase by annealing above 1173 K under an O₂ atmosphere. When the precursor obtained at T_red.=873 K was oxidized, the t′-(Ce_0.5Zr_0.5)O₂ phase identical to the starting sample appeared. The structural difference between t′_meta and t′ phases was estimated to be in their oxygen parameter, z, for the 4d site in the space group of P4₂/nmc. The oxygen parameter for the t′_meta phase is closer to 0.25 than that for t′ phase. It was shown that the temperature and holding time for precursor preparation and the oxygen content δ are important factors for the structure of the reoxidized CeZrO₄ compound.