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)O-2 phase

Cerium zirconium oxides with the composition CeZrO4 were prepared by oxidiz ing 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 phas e at 873 less than or equal to T-red less than or equal to 1573 K. The CeZr O4 compounds together with the precursors were characterized by powder X-ra y diffraction and IR and Raman spectroscopies. In the case of 1323 less tha n or equal to T-red. less than or equal to 1573 K, the precursors were iden tified as having the pyrochlore-type Ce2Zr2O7+2 delta phase with delta < 0. 06. When the reduction temperature was decreased, the ordering level betwee n Ce and Zr ions was decreased in the pyrochlore-type phase. The CeZrO4 com pounds obtained by oxidizing the pyrochlore-type precursors were identified as the kappa-CeZrO4 phase in which Ce and Zr ions are in an ordered arrang ement 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 t he 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 th e precursor obtained at T-red. = 1123 K was oxidized, a tetragonal phase ap peared, 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, b ecause 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. T he 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 tha t for t' phase. It was shown that the temperature and holding time for prec ursor preparation and the oxygen content delta are important factors for th e structure of the reoxidized CeZrO4 compound. (C) 1999 Academic Press.