Mechanisms for enhancing n-type polaronic transport in transition metal oxides: ionic size, pair formation/clustering, and valence effects

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Non-Technical Abstract This award supports a project focused on studying a group of materials called transition metal oxides. These materials are promising candidates for making devices for storing energy such as lithium-ion batteries and for converting energy such as solar cells. However, their efficiency has been fundamentally limited by the slow movement of negative and positive charges in them, restricting their use in practical applications. The main reason for the restricted movement of these charges is that they get trapped by the distortions in the materials’ atomic structure. This project will investigate the use of dopants to improve the mobility of the charges in iron oxide, a representative of these transition metal oxides. It will determine the design principles of “good” dopants that improve the transport of charges in these oxides so that they can be used in practical energy applications. The project will train undergraduate and graduate students and will engage in outreach activities involving high school students, emphasizing in broadening participation of underrepresented students.Technical Abstract This research project aims to quantitatively investigate the mechanisms that limit charge transport, including how the clustering of extrinsic n-type dopants takes place, how the interaction between electron small polarons and (clustered) dopants affects the electronic transport properties, and what steps might increase the carrier concentration in transition metal oxides. This project will initially use hematite, a representative polaronic oxide, doped with tin and other 4+/5+ ions as a model system for the study. The dopant clustering is believed to be due to ionic size mismatch, and consequently limit the carrier conductivity. The research team will use a “co-doping” strategy to improve the solubility of dopants and avoid clustering and correspondingly enhance carrier mobility as a function of doping concentration. A crucial step in this project is determining the local environment about extrinsic dopants and dopant clusters and if/how they are modified when the second dopants are incorporated; this work will combine physical measurements with first-principles simulations. The findings in this project can provide rules for rational design of polaronic oxides with improved transport properties, and more importantly, develop a fundamental understanding of key factors determining whether an isolated dopant, dopant cluster or co-doping can be beneficial or harmful for transport properties.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.