Influence of thermally induced oxygen order on mobile ion dynamics in Gd_(2)(Ti_(0.65)Zr_(0.35))_(2)O_(7)

Abstract

We report on the influence of oxygen order in the oxygen-ion dynamics in the ionic conductor Gd_(2)(Ti_(0.65)Zr_(0.35))_(2)O_(7). The metastable Gd_(2)(Ti_(0.65)Zr_(0.35))_(2)O_(7) powders prepared by mechanical milling present an anion-deficient fluorite type of structure, stable up to about 800 °C. Thermal treatments at higher temperatures facilitate the gradual rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects. Interestingly, metastable pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process. We have found that the ionic conductivity due to mobile oxygen ions increases significantly with increasing sintering temperature from 800 to 1500 °C as a result of a systematic decrease in the activation energy for the dc conductivity from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by stretched exponentials of the form φ(t)=exp[-(t/τ)^(1−n)], and the fractional exponent n decreases systematically from n=0.51 to 0.18 with increasing sintering temperature. These results are explained in terms of weaker ion-ion interactions in the increasingly ordered structure of the samples sintered at higher temperatures, and point to the importance of structural disorder in determining the dynamics of mobile oxygen ions.