Defect structure of yttria-stabilized zirconia and its influence on the ionic conductivity at elevated temperatures

Abstract

The defect structure of cubic fluorite structured yttria-stabilized zirconia $({\mathrm{ZrO}}_{2}{)}_{1\ensuremath{-}x}({\mathrm{Y}}_{2}{\mathrm{O}}_{3}{)}_{x}$ has been investigated over the composition range $0.100(4)l~xl~0.241(10)$ and temperatures $T(K)$ up to 2780(10) K, using single-crystal specimens. Analysis of neutron and x-ray diffraction data, including both Bragg and coherent diffuse scattering components, has identified three principal types of defects within the fluorite lattice. At low yttria concentrations $(xl\ensuremath{\sim}0.15)$ there are regions of the crystal \ensuremath{\sim}20 \AA{} in size which contain relatively few oxygen vacancies, causing the lattice to undergo a slight tetragonal distortion of the type observed in the tetragonal phase of $({\mathrm{ZrO}}_{2}{)}_{1\ensuremath{-}x}({\mathrm{Y}}_{2}{\mathrm{O}}_{3}{)}_{x}$ at $xl\ensuremath{\sim}0.09.$ The oxygen vacancies are preferentially arranged in pairs on nearest-neighbor anion sites in the 〈111〉 fluorite directions, with a cation located between them and extensive relaxations of the surrounding nearest-neighbor cations and anions. As the yttria content increases, these 〈111〉 vacancy pairs pack together in 〈112〉 directions to form aggregates, whose short-range defect structure resembles the long-range crystal structure of the ordered compound ${\mathrm{Zr}}_{3}{\mathrm{Y}}_{4}{\mathrm{O}}_{12}$ and other anion-deficient fluorite-related systems. The aggregates are typically \ensuremath{\sim}15 \AA{} in diameter, though both their size and number density increase slightly with x. On increasing the temperature, these aggregates remain stable up to close to the melting point. There is also an increasing number of single vacancies and 〈111〉 vacancy pairs (with surrounding relaxation fields) as x increases, and these isolated clusters become mobile at $Tg\ensuremath{\sim}1000\mathrm{K}$ and give rise to the high ionic conductivity of the material. In light of these observations, we propose that the anomalous decrease in the ionic conductivity with increasing x is a consequence of the decreasing mobility of the isolated defects, possibly due to blockage by the increasing number of static aggregates.