Crystal Chemistry of Y2(ZrySn1-y)2O7 Pyrochlore Solid Solutions and its Relation to Fast-Ion Conduction

Pyrochlore oxides, A2B2O7, are of interest as fuel cell materials and sensors because, with suitable doping, they may be fast-ion conducting or p- or n-type electronic conductors. An earlier analysis of Y2(ZryTi1-y)2O7 solid solutions showed that substitution of the larger Zr ion for Ti induced progressive disorder in both the cation and anion arrays causing a 103 increase in oxygen ion conductivity. In contrast, Y2(SnyTi1-y)2O7 solid solutions remained essentially fully ordered over their entire range of composition even though the range of average radii of the cation species in the B site overlapped to a considerable degree with those of the (ZryTi1-y) materials. The present work used Rietveld analysis of both neutron and x-ray powder profiles to examine Y2(ZrySn1-y)2O7 compositions along the third leg of the pseudo-ternary system: solid solutions between the stannate, with strong tendency to remain ordered, and the zirconate which has predilection toward complete disorder. Progressive disorder in the oxygen ion array was again observed with increasing y. Mixing between cation sites was found to progress at a rate independent of anion disordering. The behavior was very similar to Y2(ZryTi1-y)2O7, suggesting that the differing behavior of the three systems is due to the ability of Zr to readily enter the eight-coordinated A site.