Defect structure, ionic conductivity, and diffusion in yttria stabilized zirconia and related oxide electrolytes with fluorite structure

Ionic conductivity data of yttria zirconia, Y/sub 4x/Zr/sub 1-4x/O/sub 2-2x/, reported in the literature as functions of oxygen vacancy concentration, x, and temperature, T, have been analyzed according to a theoretical model developed in a previous paper and applied to calcia stabilized zirconia. Because of the different charge compensation mechanism in yttria stabilized zirconia, Y/sub 4x/Zr/sub 1-4x/O/sub 2-2x/, than that in calcia stabilized zirconia, Ca/sub 2x/Zr/sub 1-2x/O/sub 2-2x/, the theory predicts the occurrence of a maximum of ionic conductivity at an oxygen vacancy concentration, x = 0.03125 (= 1/32), which is just half the oxygen vacancy concentration at the maximum, x = 0.0625 (= 1/16), in calcia stabilized zirconia. This was clearly confirmed by recent experimental results of Ioffe et al. and appears to substantiate further the basic validity of the theoretical model. Results of the theoretical analysis on yttria stabilized zirconia obtained herein are compared with those on calcia stabilized zirconia are also discussed in relation to the ionic conductivity data of the various rare earth oxide-stabilized zirconia systems. The basic assumptions and limitations of the theoretical model are critically examined, and possible modifications of the theoretical model are suggested