Effect of La substitution for Gd in the ionic conductivity and oxygen dynamics of fluorite-type Gd2Zr2O7

We have prepared different compositions in the Gd 2− y La y Zr 2 O 7 solid solution by mechanically milling stoichiometric mixtures of the corresponding oxides. Irrespective of their lanthanum content, as-prepared powder samples consist of single-phase anion-deficient fluorite materials, although the long-range ordering of cations and anion vacancies characteristic of pyrochlores was observed for y ≥0.4 after post-milling thermal treatments at 1200 °C. Ionic conductivity was found to be thermally activated and almost independent of La content for 0≤ y ≤1, since the pre-exponential factor decreases as structural ordering increases; however, there is a concomitant decrease of the activation energy E dc for oxide-ion diffusion, from E dc = 1.13 ± 0.02 eV for the anion-deficient fluorite Gd 2 Zr 2 O 7 to E dc = 0.85 ± 0.03 eV for the partially ordered pyrochlore-type Gd 1.2 La 0.8 Zr 2 O 7 . Electrical conductivity relaxation is well described by a Kohlrausch–Williams–Watts (KWW) function of the form Φ = exp(−( t /τ) 1− n ), where the fractional exponent n decreases as the La content (ordering) increases. These results are explained in terms of weaker ion–ion interactions in the better ordered structure and highlight the importance of structural ordering/disordering in determining the dynamics of mobile oxygen ions.