Graduate Thesis Or Dissertation

 

Exploratory Studies Within the Exotic Lyonsite Structure Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x059cd73k

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  • Structure-property relationships have always underpinned the field of solid state chemistry; dependable structural descriptions are integral to understanding and controlling the physical properties of inorganic compounds old and new. While a handful of crystal structures are celebrated for their versatility, flexibility, and functionality, the existence of more exotic structures is often ignored. Without methodical study, potential solutions to contemporary material challenges may repeatedly escape notice. The lyonsite structure is one such under-studied system. Existing research has shown that some lyonsites exhibit behaviors that may be favorable for usage as inexpensive photocatalysts, wavelength-selective optical filters, or microwave dielectrics. This dissertation therefore systematically investigates the structural flexibility and physical properties of a series of lyonsite oxides. Incorporation of Co2+ into the Zn2.5– xCoxVMoO8 system was successful and the subsequent effect on the Zn site preference was explored. All samples were annealed between 700 and 1000 °C, resulting in green-colored pastilles. Powder X-ray diffraction (PXRD) and neutron powder diffraction (NPD) confirmed the solid solution range to be 0 ≤ x ≤ 2.5, with all members isostructural with the parent compound. Magnetic susceptibility measurements indicated Curie-Weiss behavior in the high T region, confirming the presence of Co2+. Diffuse reflectance measurements are also presented to explain the color mechanisms of this series. Next, a comparative investigation was carried out within the Zn2.5– xAxVMoO8 (A2+ = Mn, Ni, Cu) system. Samples were prepared between 700 and 1000 °C, resulting in various colors. All three solid solutions were found to exhibit a miscibility gap before or at the midpoint, beyond which phase separation occurs. X-ray and neutron powder diffraction were used to elucidate the cause of these solubility limits and diffuse reflectance measurements reveal varying color mechanisms. All phase pure members of the solid solutions are found to be similar to the parent compound, and magnetic susceptibility measurements confirmed the +2 oxidation state for all substitutions, with tendency towards antiferromagnetic exchange as x increases. Finally, the demonstrated cationic flexibility of this structure was exploited in an investigation of the vacancy tolerance in the A site. In the parent compound Co3.75V1.5Mo1.5O12, it is apparent that a stoichiometric vacancy of 0.25 is unavoidable, as a result of both coulombic repulsion and simple charge balance. This work focuses on the systematic elimination of said vacancies by introducing increasing Li content, with varying V content for charge balance. A full solid solution was found to exist with the formula 0.25−x/8LixCo3.75−7x/8V1.5−3x/4Mo1.5+3x/4O12 (0 ≤ x ≤ 2), terminating at the known end member Li2Co2Mo3O12. Samples were synthesized using high temperature solid state reaction methods in air under ambient pressure. Lattice refinements on PXRD data confirmed the isostructural nature of the whole series. Structure-property relationships were also explored via magnetic susceptibility, optical, and dielectric measurements.
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  • Ongoing Research
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  • 2018-11-26 to 2020-12-26

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