NSF/DMR-BSF: The Effects of Configurational Disorder on Polaron Transport

Cornell University, Ithaca NY

Investigators

Abstract

Nontechnical Abstract: Oxides are well known as materials with low electronic conductivity. But upon closer examination, there is a great deal of variability in their ability to conduct charge, and much of this is unexplained. For instance, in cobalt manganese oxides the conductivity can increase up to 10 times depending on the cobalt to manganese ratio. The existing theories are accurate when there are only two components, for instance, just cobalt and oxygen, but when a third component is included, the theories break down. In this project, the team is investigating a fundamental question of oxides: how does atomic disorder affect conductivity in oxides. By elucidating these mechanisms, the team is opening up new avenues to tailor the conductivity in oxides, important for applications that rely on the insulating properties of oxides, as in semiconductor transistors, or for applications that could benefit from increased conductivity, as in batteries and fuel cells. To disseminate and promote science to the public the team is creating a website with on-line training videos to strengthen international science outreach and the US-Israel partnership using science for peaceful purposes, and the team is designing a module on electronic conductivity in oxides for Lending Library Experiments. The kit aligns with specific Next Generation Science Standards and uses exploratory hands-on activities to help experiential learning. Technical Abstract: The purpose of this work is to investigate a fundamental question in ternary oxides: how do charge carriers move through a spinel crystal lattice when there is more than one type of cation. It is known that the most prominent mechanism of charge transport in oxides is through hopping that follows the polaron models. But the polaron hopping models are too crude to account for the configurational differences that occur in ternary transition metal spinels, where the two different cation types can exhibit large degrees of disorder between the two cation lattice sites and possess a variety of oxidation states. As a consequence, the effects of cation disorder and cation oxidation states on electronic conductivity has not been well-described in multinary spinels. To investigate this question the team is synthesizing ternary oxide spinels, characterizing the cation disorder, and correlating the disorder with the electronic transport. The team is using x-ray emission spectroscopy, high-angle annular dark-field imaging, and electron energy loss spectroscopy to determine site occupancy, oxidation states, and local atomic segregation. The experimental results are coupled with theory to understand the mechanisms and outline a global model for transport. The team is using DFT-type calculations with periodic and non-periodic boundary conditions, and is analyzing charge transport characteristics related to hopping mechanisms in oxide materials to determine if the charge carriers lie on delocalized (overlapping) or localized states for specific cation distributions and concentrations. Understanding the fundamental mechanisms of polarons is a grand challenge for fields including molecular architectures, and electrical energy storage, where details of charge transport in poor electron-conductor materials could have a large impact. Understanding the relationship between cation configurational disorder and polaron hopping could lead to designer oxides with tailored properties. The team is promoting science to the public by creating a website with on-line training videos for international science outreach and to strengthen the US-Israel partnership using science for peaceful purposes, and designing an experiment on electronic conductivity in oxides for Lending Library Experiments. 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.