The 5d transition metal oxides have drawn substantial interest for predictions of being suitable candidates for hosting exotic electronic and magnetic states, including unconventional superconductors, magnetic skyrmions, topological insulators, and Weyl semimetals. In addition to the electron-electron correlation notable in high-temperature 3d transition metal superconductors, the 5d oxides contain a large spin-orbit interaction term in their ground state, which is largely responsible for the intricate phase diagram of these materials. Iridates, or compounds containing 5d iridium bonded with oxygen, are of particular interest for their spin-orbit split Jeff = 1/2 state, which is partially filled without the presence of any additional electron correlation. However, the comparable energetics between a small, finite electron correlation energy and the spin-orbit interaction make the band structure of iridates amenable to small perturbations of the crystalline lattice and ideal for exploring the interplay between these two interactions. While altering the spin-orbit interaction strength of iridium is tenably not feasible, the electron correlation energy can be tuned using a variety of experimental techniques. In this dissertation, the electronic and magnetic properties of iridates at various electron correlation energies are studied by altering the epitaxial lattice strain, dimensionality, and the radius size of the A-site cation. These parameters tune the effective electronic bandwidth of the system, which is inversely proportional to the effective electron correlation energy. The lattice strain and the cationic radius size achieve this by altering the Ir-O-Ir bond angle between nearest neighbor Ir ions. In the case of dimensionality tuning, the effective bandwidth is controlled via the coordination number of each Ir ion. In the first study, a metal-to-insulator transition is observed in thin films of the semi-metallic SrIrO3 as in-plane compressive lattice strain is increased. This observation is consistent with the expectation of compressive lattice strain increasing the effective correlation energy; however, optical spectroscopy spectra reveal the increase is not sufficient for opening an insulating Mott gap. In the second part, the effective correlation energy is adjusted using a dimensional confinement of the layered iridate Sr2IrO4. Here, the coordination number of each Ir ion is reduced using an a-axis oriented superlattice of one-dimensional IrO2 quantum stripes, where several emergent features are revealed in its insulating Jeff = 1/2 state. In the final study, the effective correlation is tuned in a series of mixed-phase pyrochlore iridate thin films, where the Ir atoms take a corner-shared tetrahedral configuration. Here, a transition between conducting to insulating magnetic domain walls is revealed as the correlation energy is increased via A-site chemical doping. Each of these studies sheds light on the pronounced role the effective correlation energy plays in determining the local subset of phases predicted for iridates and related systems featuring strong spin-orbit interactions.

The layered iridates such as Sr2IrO4 and Sr3Ir2O7, have attracted substantial attention due to their novel electronic states originating from strong spin-orbit coupling and electron-correlation. Recent studies have revealed the possibilities of novel phases such as topological insulators, Weyl semimetals, and even a potential high-Tc superconducting state with a d-wave gap. However, there are still controversial issues regarding the fundamental electronic structure of these systems: the origin of the insulating gap is disputed as arising either from an antiferromagnetic ordering, i.e. Slater scheme or electron-correlation, i.e. Mott scheme. Moreover, it is a formidable task to unveil the physics of layered iridates due to the limited number of available materials for experimental characterizations. One way to overcome this limit and extend our investigation of the layered iridates is using metastable materials. These materials which are far from their equilibrium state, often have mechanical, electronic, and magnetic properties that different from their thermodynamically stable phases. However, these materials cannot be synthesized using thermodynamic equilibrium processes. One way to synthesize these materials is by using pulsed laser deposition (PLD). PLD is able to generate nonequilibrium material phases through the use of substrate strain and deposition conditions. Using this method, we have synthesized several thermodynamically metastable iridate thin-films and have investigated their electronic and optical properties. Synthesizing and investigating metastable iridates opens a path to expand the tunability further than the ability of the bulk methods. This thesis consists of four studies on metastable layered iridate thin film systems. In the first study, three-dimensional Mott variable-range hopping transport with decreased characteristic temperatures under lattice strain or isovalent doping has been observed in Sr2IrO4 thin films. Application of lattice strain or isovalent doping exerts metastable chemical pressure in the compounds, which changes both the bandwidth and electronic hopping. The variation of the characteristic temperature under lattice strain or isovalent doping implies that the density of states near the Fermi energy is reconstructed. The increased density of states in the Sr2IrO4 thin films with strain and isovalent doping could facilitate a condition to induce unprecedented electronic properties, opening a way for electronic device applications. In the second study, the effects of tuning the bandwidth via chemical pressure (i.e., Ca and Ba doping) on the optical properties of Sr2IrO4 epitaxial thin films has been investigated. Substitution of Sr by Ca and Ba ions exerts metastable chemical pressure in the system, which changes both the bandwidth and electronic hopping. The optical conductivity results of these thin films suggest that the two-peak-like optical conductivity spectra of the layered iridates originates from the overlap between the optically-forbidden spin-orbit exciton and the inter-site optical transitions within the Jeff = ½ band, which is consistent with the results obtained from a multi-orbital Hubbard model calculation. In the third study, thermodynamically metastable Ca2IrO4 thin- films have been synthesized. Since the perovskite structure of Ca2IrO4 is not thermodynamically stable, its bulk crystals do not exist in nature. We have synthesized the layered perovskite phase Ca2IrO4 thin- films from a polycrystalline hexagonal bulk crystal using an epitaxial stabilization technique. The smaller A-site in this compound compared to Sr2IrO4 and Ba2IrO4, increases the octahedral rotation and tilting, which enhance electron-correlation. The enhanced electron-correlation is consistent with the observation of increased gap energy in this compound. This study suggest that the epitaxial stabilization of metastable-phase thin-films can be used effectively for investigating complex-oxide systems. Finally, structural, transport, and optical properties of tensile strained (Sr1-xLax)3Ir2O7 (x = 0, 0.025, 0.05) thin-films have been investigated. While high-Tc superconductivity is predicted in the system, all of the samples are insulating. The insulating behavior of the La-doped Sr3Ir2O7 thin-films is presumably due to disorder-induced localization and ineffective electron-doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates. These studies thoroughly investigate a wide array of novel electronic and optical phenomena via tuning the relative strengths of electron correlation, electronic bandwidth, and spin-orbit coupling using perturbations such as chemical doping, and the stabilization of metastable phases in the layered iridates.

<p dir="ltr">Catalysis will play a pivotal role in the transformation of the current chemical and fuel industries, driving efforts to mitigate greenhouse gas emissions, curbing the release of hazardous waste, and efficiently utilizing energy resources. Hence, it is crucial to establish a fundamental understanding of the active sites that drive chemical reactions and the transformation of these active sites under varying reaction conditions. A particular class of catalysts that are extensively used in industrially relevant reactions, but not well understood, are metal nanoparticles supported over transition metal oxides. Under specific conditions, the metal nanoparticles are believed to be partially covered by reduced, ultrathin oxide films, which can drastically transform the physical, chemical, and electronic properties of the catalyst surface. These transformations are often referred to as the Strong Metal Support Interactions (SMSI). The structure and chemical properties of the encapsulating SMSI overlayers can determine the reactivity, selectivity, and stability of the catalyst. To explore these phenomena, the encapsulating overlayers on metal nanoparticles are most effectively studied using ultrathin film models supported on single crystal transition metal substrates. In this thesis, periodic density functional theory (DFT) calculations, along with surface science experiments in collaborators’ groups, are carried out to systematically study the molecular-level underpinnings of the metal oxide transformations.</p><p dir="ltr">As a starting point, we analyze the Pd/ZnO system. This is a potential methanol synthesis catalyst, and since ZnO is an irreducible oxide, it provides a test of the traditional hypothesis that partial reduction of support cations is necessary to exhibit SMSI. In order to compare our calculations with surface science experiments, where the ultrathin films are not in equilibrium with bulk species, we developed a mixed canonical – grand canonical phase diagram scheme. The scheme, when combined with exhaustive DFT calculations of many different ultrathin ZnO_xH_y film structures and stoichiometries, permits direct comparison of the calculated free energies of these disparate films. Although, the thin film models provide more well-defined conditions for studying SMSI, there are thermodynamic differences with the real SMSI system. These differences can be described by changing the thermodynamic ensemble used to analyze the DFT results and extrapolating to deduce the stability of films at realistic SMSI conditions. Using this formalism, we have discovered that ZnO_xH_y films on Pd, which don’t exist in bulk, may form, and promote SMSI in irreducible oxides. This behavior is traced to both hydrogen incorporation in the films and strong stabilization of the films by the Pd substrates.</p><p dir="ltr">The computational framework, initially developed for the Pd/ZnO system, is subsequently extended to conduct thermodynamic investigations across different metal substrates. We found that linear scaling relationships (SRs) exist for the ultrathin films on metal surfaces that correlate the film formation energies with the combination of oxide cation and anion binding energies. However, these SRs deviate from classic bond order conservation principles. To provide an explanation for these deviations, and to enhance the predictive capabilities of the SRs, we introduced a generalized bonding model for oxy-hydroxy films supported on metal surfaces. By combining the SRs with grand canonical phase diagrams, we can precisely predict the stability of encapsulated films under specific reaction conditions. To validate the computational scheme, we apply it to the traditional SMSI system involving TiO₂-supported metal nanoparticles. Our calculations accurately predict which metals are prone to exhibit SMSI-like behavior and align well with available experimental results.</p><p dir="ltr">In order to analyze how these structures affect important real-world chemistries and identify key descriptors that influence their reactivity, we studied the adsorption behavior of common intermediates on oxide-decorated metal surfaces. We first investigated two types of ultrathin films, the compact graphite-like ZnO and the open honeycomb-like Zn₆O₅H₅ on Pt(111). We found that the graphite-like ZnO islands barely affect the electronic properties of the Pt surface, while the honeycomb-like Zn₆O₅H₅ network tunes the surface electron density of Pt such that the binding site for CO shifts from on-top to the bridge site. The findings enhance our understanding of metal-hydroxide interactions, potentially paving the way for innovative designs of highly efficient catalytic systems.</p><p dir="ltr">The SMSI effect is not confined to oxides used as supports. We confirmed the existence of a closely related phenomenon in Pt alloys, which are an important system for the oxygen reduction reaction (ORR). We identified elements that form stable oxy-hydroxy moieties on Pt surfaces under ORR conditions. Remarkably, elements like Cr, Mo, and Ir can form stable hydroxide 0d and 2d structures on Pt and can resist dissolution by preferentially covering the Pt edge and kink sites, which are otherwise susceptible to degradation. These nanoscale structures exhibit properties different from their bulk counterparts and can effectively tune the reactivity of the surface by introducing an inhomogeneous strain field into the Pt terrace sites.</p><p dir="ltr">The overarching goal of this dissertation is to formulate design principles applicable to metal nanoparticle catalysts coated with surface oxides. Given the pivotal role of these systems in industrially significant catalysts, the development of strategies aimed at engineering novel active sites using surface oxides is of great importance. The comprehensive molecular-level understanding of metal-metal oxide interactions, established through these studies, thus serves as a foundation for the study of these effects across a wider spectrum of reactions beyond ORR and CO oxidation. Through such studies, combined with rigorous experimental confirmation, it may ultimately be possible to engineer new classes of metal/oxide interfaces for desired catalytic applications.</p>

Les concepts de matériaux multiferroïques et/ou magnétoélectriques permettent d’envisager de nouveaux dispositifs de mémoires plus performants et moins consommateurs d’énergie. Malheureusement de tels matériaux présentant ces propriétés à température ambiante ne sont pour l’instant pas disponibles. Les matériaux qui font l’objet des études présentées dans ce manuscrit, les ferrites de gallium Ga2-xFexO3, 0,7 ≤ x ≤ 1,4, sont d’excellents candidats. Le présent travail de thèse en a étudié les propriétés électriques, tant sur matériaux massifs que couches minces. Nous avons mesuré une polarisation sur composés massifs du même ordre de grandeur que celle précédemment déterminée par calculs ab initio. Une considérable réduction des courants de fuite habituellement observés en couches minces a été possible grâce à la substitution de Fe2+ par Mg2+. Une polarisation réversible et un effet magnétoélectrique ont alors pu être mis en évidence. Compte tenu du caractère ferrimagnétique à température ambiante des couches minces considérées, ceci constitue la première manifestation d’un matériau multiferroïque et magnétoélectrique à réel intérêt applicatif. / Concepts of multiferroic and magnetoelectric materials allow designing new memory devices with better performances and less energy consumption. Unfortunately, such materials with room temperature applicability are not yet available.The material concerned by this study, gallium ferrites Ga2-xFexO3, 0.7 ≤ x ≤ 1.4, are excellent candidates for such applications. This work aimed at studying the electrical properties of the bulk material, as well as in thin films. We have measured a polarization on the bulk samples comparable to the value estimated by first principle calculations. A considerable reduction of the leakage currents usually observed in oxide thin films has been achieved by the doping by substitution of Fe2+ in the structure, using Mg2+. A switchable polarization and a magnetoelectric effect at room temperature in thin films have been observed. Considering the room temperature ferrimagnetic behavior of this compound, this is the first manifestation of a multiferroic and magnetoelectric with real potential and technological applications.