(High-Temperature Energy, Materials, & Processes Division J. Bruce Wagner, Jr. Award) Impact of Structural Disorder on Opto-electro-chemo-mechanical Interactions in Mixed Conducting Perovskite Oxides

Abstract

Multifunctional mixed ionic and electronic conducting (MIEC) perovskite oxides – which chemo-mechanically interact with their environment, catalyze reactions, and selectively transport charged species – serve important functions in electrochemical energy conversion and storage devices, gas separation membranes, sensors, actuators, and reactors for chemical fuel production. Ideally these materials should exhibit rapid surface exchange kinetics and transport, for efficiency and fast responses, and tailorable chemical expansion during stoichiometry changes, for controlled dimensional changes and stresses. Therefore, key materials property metrics relating to performance include the ionic and electronic conductivities, surface exchange coefficients (k), and coefficients of chemical expansion (CCE). It is important to understand at a fundamental level how the structure of MIEC perovskite oxides at various scales impacts these key metrics so that design principles for more efficient and durable MIECs can be established. To this end, we have synthesized and characterized in situ bulk and thin film titanate, gallate, zirconate, and cerate perovskites, with aliovalent substitution, as model systems to understand how systematically varied microstructure, crystallinity, crystal symmetry, and defect chemistry impact the opto-electro-chemo-mechanically coupled behavior. In this talk, I would like to highlight in particular two recent results investigating the role of structural disorder. First, we have been developing in situ optical transmission relaxation for the study of defect equilibria and dynamics in perovskite and perovskite-related thin film mixed conductors. We have demonstrated the origins of optical absorption in a non-dilute model system, Sr(Ti,Fe)O3-x, and shown that in the conditions of our studies, the absorption at key wavelengths is proportional to the oxygen concentration. This relationship allows the fitting of in situ thin film optical relaxation curves, that result from oxygen partial pressure changes, to determine k without any current collectors and continuously. The optical absorption is also sensitive to structural changes in the films, so the technique enables simultaneous monitoring of, e.g., crystallization and k. Through this method, and in combination with X-ray absorption studies, we have observed for several compositions how atomic scale order impacts k and how k is enhanced orders of magnitude through in situ crystallization. Second, we have been investigating disorder at the crystal structure length scale to determine how distortions away from the perfect cubic perovskite structure impact the CCEs. We have seen that CCEs monotonically decrease as crystal structure distortions increase (i.e., tolerance factor deviates more from the ideal value of 1). Both microstructural and crystal chemical mechanisms for the reduced expansion for a given stoichiometry change will be presented. This work complements our earlier studies into atomistic factors that impact perovskite CCEs, including electronic charge localization and size of oxygen vacancies. Practical implications of these recent results demonstrating optical measurement and structure-based tailoring of k and CCEs will be discussed.