The defect structure of cubic yttria-stabilized material was investigated for yttria contents of 10 to 24.1mol% and temperatures of up to 2780K, using monocrystalline specimens. An analysis of neutron and X-ray diffraction data, including Bragg and coherent diffuse scattering components, revealed 3 main types of defect within the fluorite lattice. At yttria concentrations of less than 15mol%, there were regions, some 2nm in size, which contained relatively few O vacancies and caused the lattice to undergo a slight tetragonal distortion of a type which was observed in the tetragonal phase at yttria contents below 9mol%. The O vacancies were preferentially arranged in pairs on nearest-neighbor anion sites in the <111> fluorite directions, with a cation located between them; and with extensive relaxation of the surrounding nearest-neighbor cations and anions. As the yttria content increased, these <111> vacancy pairs packed together in <112> directions so as to form aggregates whose short-range defect structure resembled the long-range crystal structure of the ordered compound, Zr3Y4O12 and of other anion-deficient fluorite-related systems. The aggregates were typically about 1.5nm in diameter, although both their size and their number density increased slightly with the yttria content. Upon increasing the temperature, these aggregates remained stable almost up to the melting point. There were also increasing numbers of single vacancies and <111> vacancy pairs as the yttria content increased. These isolated clusters became mobile at temperatures above 1000K, and gave rise to the observed high ionic conductivity. It was proposed that the anomalous decrease in ionic conductivity, with increasing yttria content, was a result of the decreasing mobility of isolated defects; possibly due to blockage by the increasing number of static aggregates.

Defect Structure of Yttria-Stabilized Zirconia and its Influence on the Ionic Conductivity at Elevated Temperatures. J.P.Goff, W.Hayes, S.Hull, M.T.Hutchings, K.N.Clausen: Physical Review B, 1999, 59[22], 14202-19