Connection between oxygen-ion conductivity of pyrochlore fuel-cell materials and structural change with composition and temperature

Oxides are believed to assume the A2B2O7 pyrochlore structure type for a specific range of ratios of the cation radii, RA/RB. Substitution of a larger B′ ion in solid solution for B can progressively drive the system to complete disorder as in Y2(Zry Ti1−y)2O7, producing an oxygen-ion conductivity, σ, greater than 10−2 S/cm at 1000°C, comparable to the values of 10−1 S/cm found for M3+-stabilized cubic zirconias. Rietveld analyses of neutron and X-ray powder diffraction data have been employed to obtain structural data for the related systems Y2(SnyTi1−y)2O7, Y2(ZrySn1−y)2O7, Gd2(SnyTi1−y)2O7 and (SczYb1−z)2Ti2O7 to test whether the state of disorder and attendant ionic conductivity are indeed determined by RA/〈RB′,RB〉 This was not the case for the Sn–Ti solid solutions: they retained an ordered pyrochlore structure for all values of y. The slight variation of ionic conductivity (less than one order of magnitude with a maximum in σ at intermediate y) was successfully explained by the structural data. The behavior of Y2(ZrySn1−y)2O7 solid solutions was very similar to that of the Zr–Ti phases. Neutron powder diffraction profiles were recorded as fully-ordered Y2Sn2O7 and highly-disordered Y2(Zr0.6Ti0.4)2O7 were heated in situ at temperatures in the range 20–1500°C. The structure of Y2Sn2O7 steadfastly remained fully-ordered over this temperature range. The principal change in the structure was increase in the positional coordinate, x, for O(1), corresponding to increased distortion of the oxygen-ion array as temperature was increased, a consequence of greater thermal expansion of the A3+–O bond relative to change in the B4+–O separation. The highly-disordered cation arrangements in Y2(Zr0.6Ti0.4)2O7 remain unchanged up to 1250°C when the oxygen array began to undergo further disorder. The same anion site occupancies were observed during heating and cooling cycles suggesting that their distribution does represent an equilibrium state.