It was recalled that oxides were believed to adopt the A2B2O7 pyrochlore structure type over a specific range of ratios of the cation radii. The substitution of a larger B-ion could gradually drive the system towards complete disorder, as in the case of Y2(Zr,Ti)2O7, and produce an O-ion conductivity that was greater than 0.01S/cm at 1000C. Rietveld analysis of neutron and X-ray powder diffraction data was used to obtain structural data for the present systems in order to test whether the state of disorder and resultant ionic conductivity were truly determined by the radius ratio. This was not the case for Sn-Ti solid solutions. They retained an ordered pyrochlore structure for all Sn contents. A slight variation in the ionic conductivity (less than an order of magnitude, with a maximum at intermediate Sn contents) could be explained by the structural data. The behaviour of the Zr-Sn solid solutions was very similar to that of the Zr-Ti phases. Neutron powder diffraction profiles were recorded while fully-ordered Y2Sn2O7 and highly-disordered Y2(Zr,Ti)2O7 were heated in situ at 20 to 1500C. The structure of Y2Sn2O7 remained fully ordered over this temperature range.

Connection between Oxygen-Ion Conductivity of Pyrochlore Fuel-Cell Materials and Structural Change with Composition and Temperature. B.J.Wünsch, K.W.Eberman, C.Heremans, E.M.Ku, P.Onnerud, E.M.E.Yeo, S.M.Haile, J.K.Stalick, J.D.Jorgensen: Solid State Ionics, 2000, 129[1-4], 111-33