Ambient pressure synthesis and characterization of strongly correlated oxides

Abstract

Strongly correlated oxides are an important class of materials with a vast array of intriguing optical, electrical, electrochemical, dielectric, and catalytic properties which makes them extremely attractive for a variety of applications, including supercapacitors, batteries, fuel cells, sensors, catalysts, solar cells, and smart windows. Their unique properties arise from the strong coupling between the d band electrons as well as electron-lattice interactions. One well-studied but not well-understood property is the insulator-to-metal transition (IMT) that can occur due to temperature, doping, and other external factors, which can be exploited for many advanced optoelectronic applications. Therefore, the central aim of this dissertation is to use a suite of electrochemical, electronic, and spectroscopic techniques to elucidate the defect-property-function correlation that underpins the metal-insulator phase transition seen in two types of correlated oxides: i) Mott-Hubbard insulators, such as VO2; and ii) charge-transfer insulators, such as rare-earth doped nickelates (RNiO3). In the first study (Chapter 3), the effects of oxygen over-stoichiometry on the band structure and IMT in VO2 are presented. Systematic modulation of VO1.86 to VO2.44 shows that charge fluctuation in the metallic phase of VO2 forms electron (e) and hole (h+) pairs, which leads to delocalized V3+ and V5+ states. As a result, the IMT is linked to changes in the V-O bond length, localization of V3+ e at V5+ sites, formation of V4+-V4+ dimers, and removal of π^* screening electrons. In addition, we show that phase transitions are linked to the lattice V3+/V5+ concentration and that electronic transitions are regulated by charge fluctuation, charge redistribution, and structural transition. In the second study (Chapter 4), a three-step phase transition in VO2 is presented as a result of high charge injection. Using two-dimensional crystalline platelets, electron doping nearly two orders of magnitude larger than previously reported is achieved using Li+. As a result of this massive charge injection, a three-step insulator-to-metal-to-insulator-to-metal transition and switch in the electrical polarity from n-type to p-type is reported for the first time. A “lattice redox model” is proposed to aid in explaining the origin of the thermal-, electrochemical-, and compositional-induced IMT, which involves V redox-induced band filling, structural distortion, and electron effects. In the third study (Chapter 5), an aging technique is presented to stabilize the Ni3+ cations in rare-earth doped nickelates at ambient pressure. A solid-state synthesis method is used followed by two types of aging: i) a slow ambient-air aging for 6-8 months or ii) an accelerated aging at a higher temperature of 650°C for 1-3 weeks. This technique was used to successfully synthesize bulk SmNiO3 and NdNiO3. Rietveld refinement showed the composition of NdNiO3 and SmNiO3 to be as high as 96 wt% and 43 wt% after aging, respectively. Sharp IMT are seen in both SmNiO3 and NdNiO3 after aging of 2-3 orders of magnitude, which is similar to high-pressure synthesized samples reported in the literature.