A NOVEL TITANATE-BASED OXYGEN-ION CONDUCTOR - GD2TI2O7

Electrical conductivity measurements are reported on the rare-earth pyrochlore compound Gd2Ti2O7 as a function of temperature, oxygen partial pressure and aliovalent dopant concentration. Doping experiments were performed on both the Gd and Ti cation sublattices with doping levels of 0.0 to 15.0 mol% cation substitution. For Ca doped (Gd1-xCax)(2)Ti2O7, the ionic conductivity, sigma(i), was observed to increase over 2 orders of magnitude with increasing x reaching a maximum value of 5 x 10(-2) S/cm at 1000 degrees C and x = 0.10. This value represents the highest ionic conductivity reported to date for a titanate based material. Accompanying the increase in sigma(i) was a corresponding decrease in ionic activation energy E(i) which dropped from 0.94 to 0.63 eV in the composition range of x = 0.00 to x = 0.02. Similar increases in sigma(i) and decreases in E(i) were evident for ''B'' site Al doping in Gd-2(Ti1-yAly)Ti2O7 but valid only up to a doping level of y = 0.01. At higher dopant levels, sigma(i) dropped sharply when the solubility limit was exceeded, as confirmed by X-ray diffraction lattice parameter measurements. The composition dependence of the ionic conductivity for both ''A'' and ''B'' site acceptor dopants are discussed in terms of defect interactions leading to favorable transport paths.