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 fo r 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 Ti1-y)(2)O-7, producing an oxygen-ion c onductivity, sigma greater than 10(-2) S/cm at 1000 degrees C. comparable t o the values of 10(-1) S/cm found for M3+-stabilized cubic zirconias. Rietv eld analyses of neutron and X-ray powder diffraction data have been employe d to obtain structural data for the related systems Y-2(SnyTi1-y)(2)O-7, Y- 2(ZrySn1-y)(2)O-7, Gd-2(SnyTi1-y)(2)O-7 and (SczYb1-z)(2)Ti2O7 to test whet her the state of disorder and attendant ionic conductivity are indeed deter mined 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 sli ght variation of ionic conductivity (less than one order of magnitude with a maximum in sigma at intermediate y) was successfully explained by the str uctural data. The behavior of Y-2(ZrySn1-y)(2)O-7 solid solutions was very similar to that of the Zr-Ti phases. Neutron powder diffraction profiles we re recorded as fully-ordered Y2Sn2O7 and highly-disordered Y-2(Zr0.6Ti0.4)( 2)O-7 were heated in situ at temperatures in the range 20-1500 degrees C. T he structure of Y2Sn2O7 steadfastly remained fully-ordered over this temper ature range. The principal change in the structure was increase in the posi tional coordinate, x, for O(1), corresponding to increased distortion of th e oxygen-ion array as temperature was increased, a consequence of greater t hermal expansion of the A(3-)-O bond relative to change in the B4+-O separa tion. The highly-disordered cation arrangements in Y-2(Zr0.6Ti0.4)(2)O-7 re main unchanged up to 1250 degrees C when the oxygen array began to undergo further disorder. The same anion site occupancies were observed during heat ing and cooling cycles suggesting that their distribution does represent an equilibrium state. (C) 2000 Elsevier Science B.V, All rights reserved.