A novel titanate-based oxygen ion conductor: Gd 2Ti 2O 7

Abstract

Electrical conductivity measurements are reported on the rare-earth pyrochlore compound Gd 2Ti 2O 7 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 (Gd 1 − x Ca x ) 2Ti 2O 7, the ionic conductivity, σ i , was observed to increase over 2 orders of magnitude with increasing x reaching a maximum value of 5 × 10 −2 S/cm at 1000 °C and x = 0.10. This value represents the highest ionic conductivity reported to date for a titanate based material. Accompanying the increase in σ 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 σ i and decreases in E i were evident for “B” site Al doping in Gd 2(Ti 1 − y Al y ) 2Ti 2O 7 but valid only up to a doping level of y = 0.01. At higher dopant levels, σ 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.