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

Oxides are believed to assume the A(2)B(2)O(7) pyrochlore structure type for a specific range of ratios of the cation radii, R(A)/R(B). Substitution of a larger B' ion in solid solution for B can progressively drive the system to complete disorder as in Y(2)(Zr(y) Ti(1-y))(2)O(7), producing an oxygen-ion conductivity, sigma greater than 10(-2) S/cm at 1000 degrees C. comparable to the values of 10(-1) S/cm found for M(3+)-stabilized cubic zirconias. Rietveld analyses of neutron and X-ray powder diffraction data have been employed to obtain structural data for the related systems Y(2)(Sn(y)Ti(1-y))(2)O(7), Y(2)(Zr(y)Sn(1-y))(2)O(7), Gd(2)(Sn(y)Ti(1-y))(2)O(7) and (Sc(z)Yb(1-z))(2)Ti(2)O(7) to test whether the state of disorder and attendant ionic conductivity are indeed determined by R(A)/(R(B),R(B)) 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 sigma at intermediate y) was successfully explained by the structural data. The behavior of Y(2)(Zr(y)Sn(1-y))(2)O(7) solid solutions was very similar to that of the Zr-Ti phases. Neutron powder diffraction profiles were recorded as fully-ordered Y(2)Sn(2)O(7) and highly-disordered Y(2)(Zr(0.6)Ti(0.4))(2)O(7) were heated in situ at temperatures in the range 20-1500 degrees C. The structure of Y(2)Sn(2)O(7) 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 A(3-)-O bond relative to change in the B(4+)-O separation. The highly-disordered cation arrangements in Y(2)(Zr(0.6)Ti(0.4))(2)O(7) remain unchanged up to 1250 degrees 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. (C) 2000 Elsevier Science B.V, All rights reserved.