STRUCTURE PROPERTY RELATIONS IN PYROCHLORE TYPE OXIDES FOR FUEL CELL APPLICATIONS. Per Önnerud, Kevin Eberman and Bernhardt J. Wuensch, Dept. of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge MA 02139

Pyrochlore oxides can exhibit high ionic conductivity or high electronic conductivity or both. They therefore have potential as Solid Oxide Fuel Cell (SOFC) materials. For the electrolyte materials of the SOFC a high ionic and a low electronic conductivity is a necessity while the cathodes require a mixed ionic and electronic conductivity. Service of the same structure in both functions would avoid materials compatibility problems.

The pyrochlore structure, A₂B₂O₇, is a superstructure of the fluorite type, (A,B)₄O₈, having twice the lattice parameter a=2a', spacegroup Fd3m. It was earlier suggested that the ratio r_A/r_B of the larger A cation and the smaller B cation is the governing parameter for the order in these systems. The pyrochlore structure can exhibit mainly two types of disorder: anti-site disorder of the cations and a Frenkel-like disorder on the oxygen-ion array. Neutron Rietveld analyses of the

Y₂(Zr_yT_1-y)₂O₇ (YZT) system have shown that substitution of zirconium for the smaller titanium ion causes disordering of both the cation and anion arrays to a non-stoichiometric fluorite-type structure. As a consequence, the ionic conductivity increases. However, for a similar series of compounds, Y₂(Sn_yTi_1-y)₂O₇, with y's chosen to match the average radius of the B site in the YZT series, no disordering is observed. Although the cations partially disorder for the end member Y₂Sn₂O₇, no anion disorder is observed. A parameter very sensitive to the degree of disorder seems to be the positional coordinate (x>=0.375) of the oxygen occupying the 4Bf position, which in a completely disordered structure becomes x=0.375.

The effect of the change in the state of disorder with composition has been used to interpret the variation in measurements of ionic conductivity obtained through impedance spectroscopy.