Synthesis of novel compounds with cation ordered fluorite and α-PbO2 related structures by oxidation of Sn2Nb2O7pyrochlore

Sn(II)(1.2)(Nb(V)(1.6)Sn(IV)(0.4))O-6 pyrochlore precursor was oxidized at temperature of the range 573-973 K in 1% O-2/Ar and 02 gases for various periods of time. Two kinds of novel metastable phases with a composition of Sn(IV)(0.6)(Nb(V)(0.8)Sn(IV)(0.2))O-3.6 could be synthesized. Further, the other novel metastable phase with the same composition was found as a phase contained. One of the metastable phases was the cubic K-CeZrO4 related-type possessing the fluorite-related structure, which was formed by the cation diffusionless insertion of the oxygen atom into original oxygen vacant site of the pyrochlore-type structure. Another was an orthorhombic alpha-PbO2 related-type possessing a cation ordered arrangement unlike a well known NiWO4 Structure. The other was the rutile related-type possessing a cation ordered arrangement. Appearance of the two latter metastable phases could be attributed to the displacement of the oxygen stacking in the K-CeZrO4 related-type phase without cation redistributions. The appearance mechanisms were analogous to the well known transformations for AX(2) compounds among rutile-type, alpha-PbO2-type, and fluorite-type phases under high pressure and its release. The dependence of the appearance of these novel metastable phases on oxygen partial pressure and temperature has been discussed in terms of the driving forces and energy barriers for reactions. (C) 2003 Elsevier B.V. All rights reserved.