(High Temperature Materials Division J. Bruce Wagner, Jr. Award Address) Disorder and Transport in Pyrochlore Oxides

Abstract

Numerous avenues are currently being explored to enhance ionic conductivity in solid oxide materials, including dopants, nano-structuring, and strain engineering. Their effect on material properties, specifically mobile carrier concentration and mobility, needs to be understood and ultimately exploited to develop advanced functional materials. Crystalline disorder in complex oxides can have a pronounced effect on mass transport, but the role of disorder within the complex landscape of material and microstructural defects is not uniquely understood.In this work, we seek to explore the relationship between intrinsic disorder and oxygen ion conduction. We are using radiation damage, not to study the fundamental damage response, but rather as a tool to induce and control the extent of disorder in oxide materials. The pyrochlore compounds, A2B2O7, consist of both ordered cation and anion sublattices. The introduction of anti-site disorder, where the A and B cations switch positions, is accompanied by increasing disorder on the anion sublattice, and complete randomization of the A- and B-site cations results in the defect fluorite phase. Several workers have previously studied the influence of structural disorder on ionic conductivity by introducing disorder through dopants, temperature, or pressure. It has previously been suggested that disorder increases carrier concentration by freeing the fixed vacancy from the ordered anion sublattice of the pyrochlore structure, while order, on the other hand, may serve to increase mobility by providing a preferential ion transport pathway. This is in contrast to recent modeling results that indicate that both carrier concentration and mobility increase with disorder. In the present work we have chosen to investigate the end members of the Gd2(ZrxTi1-x)2O7 system in order to gain a better understanding of the interplay between disorder and ionic transport. Gd2Zr2O7 is an intrinsic fast-ion conductor that is also very radiation tolerant, whereas Gd2Ti2O7 is a poor ionic conductor that is much more susceptible to amorphization under irradiation than the zirconates. Electrochemical Impedance Spectroscopy is used to measure the conductivity of materials with varying degrees of irradiation-induced structural disorder in order to better understand how the underlying transport mechanisms depend on crystalline disorder. Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.