Graduate Thesis Or Dissertation
 

Substitution Effects on the Structure and Electronic Behavior of Ternary Precious Metal Oxides

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  • Due to the high cost of precursor materials, complex oxides of 4d and 5d transition metals are under-studied compared to their 3d counterparts. Recent studies have shown that oxides containing heavier transition metals can exhibit exotic electronic states due to presence of strong spin-orbit coupling. The goal of this dissertation is to investigate and to learn more about the chemical and electronic behavior of Ir and Rh. Four oxide systems were prepared and studied: Ba₂In₂₋ₓIrₓO₅₊δ, Ba₂₋ₓLaₓInIrO₆, BaLaIn₁₋yCayIrO₆, and A₁₊ₓRh₂₋ₓO₄ (A²⁺ = Co, Ni, Cu). The solid solution series Ba₂In₂₋ₓIrₓO₅₊δ (x = 0 - 1.4, 2) was synthesized and its structural, magnetic, and charge transport properties were measured. With increasing Ir content, three transitions in the room temperature structure were observed: orthorhombic to tetragonal, tetragonal to cubic, and cubic to a monoclinic distortion of a hexagonal BaTiO₃ structure. Neutron diffraction refinements showed Ba₂In₁.₆Ir₀.₄O₅.₄ was cubic and Ba₂InIrO₆ was monoclinic. The latter result contradicts previously published XRD refinements. Magnetization measurements show Curie-Weiss behavior for x = 0.2–0.6, which arises from near 50:50 ratio of Ir(V) and Ir(VI). To our knowledge, this is the first time Ir(VI) has been stabilized with standard solid state methods under ambient conditions. Electrical resistivity measurements show all the compounds studied were semiconducting, and that resistivity decreases with increasing Ir content which suggests proximity to a metal-insulator transition. A sign reversal in the high-temperature Seebeck coefficient is observed indicating both electron and hole charge transport. Two novel solid solution series, Ba₂₋ₓLaₓInIrO₆ (x = 0–1.0) and BaLaIn₁₋yCayIrO₆ (y = 0–1.0), were prepared and several changes in structure, magnetic moment, and charge transport were observed. The Ba₂₋ₓLaₓInIrO₆ series exhibits a transition from a 6M polytype to an orthorhombic perovskite structure with increased La content whereas the BaLaIn₁₋yCayIrO₆ series transitioned from a disordered orthorhombic perovskite to an ordered cubic perovskite with increased Ca content. Seebeck measurements for both systems showed that Ir(IV)-rich compounds tended to have a n-type conduction mechanism while Ir(V)-rich compounds appeared to be p-type. Both systems were found to be semiconducting and the magnitude of the resistivity is dependent on the crystal structure and Ir environment. Magnetic measurements show that the μeff values for both systems are significantly less than predicted for Ir(IV) (1.73 μB) and greater than predicted for Ir(V) (0 μB). These results are compared to other iridate compound families. The spinels A₁₊ₓRh₂₋ₓO₄ (A²⁺ = Co, Ni, Cu) were previously reported but information on their structures and magnetic behavior was lacking. CoRh₂O₄, CuRh₂O₄, and the novel composition Ni₁.₂₅Rh₁.₇₅O₄ were prepared using standard solid state methods and characterized using diffraction and magnetic susceptibility techniques. CoRh₂O₄ was found to have the cubic spinel structure (Fd3̄m) whereas CuRh₂O₄ and Ni₁.₂₅Rh₁.₇₅O₄ crystallized in tetragonally distorted spinel structures (I4₁/amd) due to Jahn-Teller effects. Antiferromagnetic behavior was observed in χ(T) data for CoRh₂O₄ (TN = 24.9(1) K, μeff = 4.42(1) μB) and CuRh₂O₄ (TN = 24(1) K, μeff = 1.97(1) μB) and their magnetic structures while determined be an antiferromagnetic A-type and an ab-plane helical structure, respectively. An upward deviation from Curie-Weiss behavior in the χ(T) data and a positive θ value for Ni₁.₂₅Rh₁.₇₅O₄ suggests the presence of competing ferromagnetic and antiferromagnetic correlations. A divergence in ZFC and FC χ(T) data was observed for Ni₁.₂₅Rh₁.₇₅O₄ as well as a lack of long-range ordering in low temperature neutron data both of which suggest spin-glass behavior.
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  • 2017-08-09 to 2018-03-13

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