Graduate Thesis Or Dissertation
 

Tuning Functional Properties of Pyrochlore and Related Oxides via Cation and Anion Substitutions

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  • Inorganic structures play an important role in materials due to their versatility and diversity. A complete understanding of the structure of a material is vital to enhance the creation of new materials that fill voids in research. The rational design of these compounds is driven by the exploitation of structure-property relationships, resulting in optimized materials and improved devices. This dissertation navigates this interplay for pyrochlore oxides, investigating the effect of substitution on the structure and characterizing the resulting properties. The fundamentals of inorganic chemistry are presented as an introduction to the research of iridium, ruthenium, bismuth and tin containing pyrochlore materials outlined by this dissertation. The 4d and 5d transition metals have received recent attention as potential topological insulators. This property is driven by the spin-orbit interaction that increases with atomic number. This led to the substitution of various cations in the general pyrochlore formula Bi2-xMxIr2O6+y (M = Ca2+, Co2+ and Cu1+), as well as phases with related structures: Ca2Ru2O7 and Bi2-xCaxRu2-xIrxO6+y. These samples were synthesized via solid state and hydrothermal reactions, resulting in materials with similar structures and properties. In the calcium containing iridium pyrochlore Bi2-xCaxIr2O6+y, the limit of substitution was found to be ~x = 1.0, where the calcium was found to shift from the ideal position to occupy the higher multiplicity 96g site, while pairing with an oxygen vacancy. For higher calcium concentration samples calcium occupied both the 16d and 96g sites. The samples were found to be paramagnetic and metallic. In the related sample Ca2Ru2O7, in the absence of the shared A site, the calcium was found to occupy the ideal site. Cobalt substituted samples were found to have a limit of substitution of ~x = 0.5, and magnetic susceptibility was measured. Copper containing samples (Bi2-xCuxIr2O6+y) were found to have a limit of substitution of ~x = 0.5, and a similar structure to the calcium containing samples. In this preliminary neutron refinement, the copper was found to shift to the 96g position from the ideal 16d, occupying both positions at higher copper content. The oxygen sites were practically fully occupied for both the x = 0.2 and 0.4 samples, suggesting the presence of Cu1+ in the system. Further analysis is required to determine the presence and valency of both iridium and copper in the system. As with the calcium analogue, the samples are paramagnetic and metallic. The Bi2-xCaxRu2-xIrxO6+y system showed a limit of substitution of ~x = 1.0. Both the x = 0.5 and 1.0 samples were paramagnetic. Sulfur and Fluorine substituted Sn2Nb2O7 pyrochlore pigments were synthesized to produce brilliant red/orange compounds as an environmentally friendly alternative to other toxic, inorganic, red/orange pigments. Neutron diffraction structure refinements confirmed that the compositions crystallized in the pyrochlore structure, showing deviations from ideal stoichiometries with a large number of vacancies and severe local structure distortions at the A and O' sites. Chemical analysis confirmed the presence of sulfur and fluorine in the pyrochlore structure and 119Sn Mössbauer analysis confirmed the presence of both Sn2+ and Sn4+ in these compounds. Optical characterization was performed through absorbance, reflectance, and color meter measurements. The compounds were found to have ~60-95% reflectance, having added functionality as “cool” pigments. The estimated band gap was determined to be in the range of 2.35 to 2.05 eV, decreasing with increasing sulfur content, consistent with the color change from yellow to orange/red. Sn2TiNbO6F1-xClx phases were made from x = 0-1.0, resulting in increasing lattice parameter and increasing band gap with chlorine substitution. This solid solution can also potentially find application as a “cool” pigment. The solid solution Sn2Ti1-xCrxWO7-xFx was synthesized from x = 0-1.0, resulting in a decreasing lattice parameter upon chromium addition. The samples were paramagnetic, with magnetic moments from 3.108 μB to 3.513 μB for x = 1 to 0.2. Many materials in the general series Bi2-xCaxM2-xM'xO7 (M = Ti, Hf, Sn; M' = Sb, Nb, Ta) were prepared for the first time and were characterized via their structure and dielectric properties resulting in extremely low loss dielectric materials with dielectric constants from 16-98. The materials were both largely temperature and frequency independent. The lattice parameters of the quaternary pyrochlores in the series BiCaMM'O7 showed an expected linear relationship with the ionic radii of the substituted elements. The solid solution Bi2-xCaxSn2-xSbxO7-y, was analyzed via neutron diffraction, where the limit of the solid solution was close to ~x = 1.3. Until this value, the compounds crystallize in the pyrochlore phase, forming a mixture of the pyrochlore phase and the Ca2Sb2O7 weberite phase beyond this value. The refined formulas for the nominal calcium content x = 0.5, 1.0, and 1.5 in this solid solution were Bi1.36Ca0.64Sn1.60Sb0.40O6.88, Bi0.82Ca1.18Sn1.10Sb0.90O6.86, and Bi0.76Ca1.24Sn1.06Sb0.94O6.85 respectively.
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  • Ongoing Research
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  • 2018-03-20 to 2018-11-06

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