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′-(Ce0.5Zr0.5)O2 phase

Cerium zirconium oxides with the composition CeZrO4 were prepared by oxidizing in O-2 at 873 K precursors with the composition CeZrO3.5+delta (delta < 0.3). The precursors were prepared by reducing the t'-(Ce0.5Zr0.5)O-2 phase at 873 less than or equal to T-red less than or equal to 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 less than or equal to T-red. less than or equal to 1573 K, the precursors were identified as having the pyrochlore-type Ce2Zr2O7+2 delta phase with delta < 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 kappa-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 K phase was not the same as that of the pyrochlore-type structure. In the case of T-red less than or equal to 1123 K, cubic CaF2-related-type (Ce0.5Zr0.5)O(3.5+delta)/2 phases with 0.1 < delta < 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)'-(Ce0.5Zr0.5)O-2 phase, because it transformed into the t' phase by annealing above 1173 K under an O-2 atmosphere. When the precursor obtained at T-red. = 873 It was oxidized, the t'-(Ce0.5Zr0.5)O-2 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 P4(2)/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 delta are important factors for the structure of the reoxidized CeZrO4 compound. (C) 1999 Academic Press.