Ph.D. (Doctor of Philosophy)
Department of Physics
This work is an academic exploration of the capacity to manipulate the characteristics of strongly correlated electron systems, specifically transition metal perovskite oxides, through chemical substitutions and structural modifications. The high flexibility of the perovskites offers prospects for customizing materials with specific functionalities, thereby holding promise for the advancement of innovative technologies in diverse industries, ranging from electronics and energy to information storage and spintronics. My research focuses on a comprehensive overview of investigations I conducted on three specific compositions, namely alkalineand rare-earth manganese-based and nickel-based oxides, denoted as (Sr, Ba)(Mn, T i)O3−δ, (Y, T b)MnO3+δ, and (Sr, La)N iO3−δ. The objective of studying the (Sr, Ba)(Mn, T i)O3−δ system was to achieve a detailed understanding of their synthesis in their polycrystalline form, which due to its complexity has not been previously reported by any other research group despite the materials existence in single crystal form for a number of years. A thorough analysis of physical and structural properties explains previously unknown reasons for the observed differences in behavior compared to the undoped parent system, such as the absence of oxygen-vacancy-ordered phases, for example. Meanwhile, the hexagonal system (Y, T b)MnO3+δ is a potential candidate for multiferroicity. Hence, investigating the stability of this hexagonal phase in which large rare-earth Tb is partially substituted for Y at the A-site was necessary for understanding the system’s ferroelectric and magnetic properties. Specifically, our goal was to investigate the competing mechanisms of ferroelectricity when combining Y MnO3 and T bMnO3 whose ferroelectric properties are of different origins. Furthermore, we aimed to investigate the impact of rare-earth substitution and oxygen off-stoichiometry on the solid solution’s magnetic properties. To the best of our knowledge, there are no existing reports on the effects of excess oxygen on the magnetic properties of hexagonal rare-earth manganese oxides. Lastly, the study of (Sr, La)N iO3−δ system was undertaken in the hope of achieving superconductivity in the bulk version of these materials. All of these compositions were synthesized using elaborate solid-state synthesis and floating zone crystal growth techniques and underwent extensive characterization including x-ray and in-situ neutron powder diffraction in addition to routine magnetic, resistive, and specific heat measurements. For instance, isothermal redox experiments were performed on polycrystalline samples of (Sr, Ba)(Mn, T i)O3−δ, accompanied by neutron diffraction and thermogravimetric analysis. In the case of (Sr, Ba)(Mn, T i)O3−δ, particular attention was dedicated to replicating and elucidating some of the enigmatic synthesis steps made complex by the fact that two polymorphs, hexagonal and cubic, compete during the synthesis, thus necessitating the use of alternating reducing and oxidizing atmospheres to stabilize the desire phase. The successful synthesis of these materials requires first their production in their hexagonal form followed by decomposition of part of this precursor phase in hydrogen-rich atmospheres. In situ neutron diffraction redox experiments of this hexagonal phase reveal previously unknown reversible properties. The in situ neutron diffraction study of cubic (Sr, Ba)(Mn, T i)O3−δ, on the other hand, focused on understanding the lack of vacancy-ordered sublattices similar to those previously observed in their Ti- and Ba-free counterparts. Our investigations highlight the significant influence of oxygen off-stoichiometry on the physical properties of hexagonal Y1−xT bxMnO3+δ compositions (with x up to 50%). This work sheds light on the intricate relationship between oxygen intercalation and the structural, magnetic, dielectric, and electronic properties. We demonstrate that the magnetic and ferroelectric properties of this solid solution materials can be effectively tuned by the size effect using the large rare-earth Tb substituent and oxygen off-stoichiometry. Specifically, a ferromagnetic ground state emerges with a relatively high ferromagnetic Curie temperature at the expense of a complex antiferromagnetic structure that forms due to geometric frustration in the oxygen-stoichiometric parent materials Y1−xT bxMnO3.0. Relaxor-type ferroelectric properties are observed and discussed. In the La1−xSrxN iO3−δ system, achievement of a superconducting order in thin films has been attributed to the crucial role of severe oxygen reduction, leading to monovalent N i1+- oxidation state. However, the absence of comparable properties in the bulk compositions still poses an enigma. Recent publications have suggested that synthesis of high-quality precursor samples is necessary to improve the stability of the reduced nickelates, thereby mitigating potential extrinsic hindrances to the formation of superconductivity, including impurities or inferior-quality materials. Indeed, the properties of our studied compositions were found to be highly sensitive to structural changes induced by variations in A and/or B-site substitutions and oxidation/reduction, underscoring their potential for fine-tuning and optimizing properties for various applications. Our investigations emphasize the need to produce high-quality precursor crystals within the accessible doping range. The targeted functionality of these materials remains the realization of superconducting order and the expansion of the substitution range of its limited phase diagram.
Krivyakina, Elena, "Strongly Correlated Electron Systems: Rare- and alkaline-Earth Manganese- and Nickel-Based Perovskite Oxides" (2023). Graduate Research Theses & Dissertations. 7333.
Northern Illinois University
Rights Statement 2
NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.