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

Bernhardt J. Wuensch, Kevin W. Eberman, Catherine Heremans, Esther M. Ku, Per Onnerud, Evangeline M E Yeo, Sossina M. Haile, Judith K. Stalick, James D. Jorgensen

Research output: Contribution to journalArticle

344 Scopus citations

Abstract

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(ZryTi1-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.

Original languageEnglish (US)
Pages (from-to)111-133
Number of pages23
JournalSolid State Ionics
Volume129
Issue number1
DOIs
StatePublished - Jan 1 2000

ASJC Scopus subject areas

  • Chemistry(all)
  • Materials Science(all)
  • Condensed Matter Physics

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