Designing Functional Materials Based on Pyrochlore Oxides: The Role of Mixed Valence and Complex Stoichiometry

Solid state inorganic oxides play an important role in next generation electronics based on their stability, versatility, and enhanced properties over other materials. However, in order to design or tune these materials for a desired functionality it is necessary to fully understand the structure behind the properties exhibited. This dissertation explores the structure-property relationships within pyrochlore oxides through detailed structure refinements using high resolution neutron and synchrotron X-ray diffraction data as well as a thorough and systematic exploration of the resultant properties. Osmium containing oxides are rare due to the difficulty in stabilizing complex structures with fixed stoichiometry and the metastability of the phases. Bismuth substituted thallium osmate pyrochlore samples, Tl2-xBixOs2O7-y, were synthesized using solid state reactions where the solubility limit was found to be approximately x = 1.4. Members of this solid solution were characterized by their structural, electronic, magnetic, and thermal properties to understand the influence of Bi3+ substitution on the ground state. The Os containing pyrochlores crystallize in the ideal cubic pyrochlore structure (̅3), and the lattice parameter a was found to slightly increase as a function of Bi content. A possible interplay between structure and cation valence states was explored using both neutron powder diffraction and X-ray absorption spectroscopy, suggesting that a combination of Os4+/Os5+ and Tl1+/Tl3+ mixed valency throughout the solid solution allows for the stabilization of the pyrochlore structure. The system is metallic for the entire solid solution and predominantly exhibits temperature independent paramagnetic behavior. Specific heat measurements show an enhanced Sommerfeld coefficient, a possible flat-band signature. This system gave insight into the bonding preferences of Os, indicating a dependence on high oxidation states and mixed valence for the stability of complex structures. Next, solid solutions with the formula of Bi2-xA’xRu2O7-y (A’ = Mg, Ca, Sr; 0 ≤ x ≤ 0.2 for Mg, 0 ≤ x ≤ 1 for Ca, and 0 ≤ x ≤ 0.5 for Sr) were synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group ̅3 with complex A/A’ cation coordination environments. The Bi cation is found to be off-center from the ideal position due to a lone pair distortion, while the positions of substituted A’ cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 Å. Interestingly, this is one of only two known pyrochlores with Mg2+ located in an 8 coordinated site. The average Ru oxidation state for each substitution is found to increase, charge compensating for the lower divalent A’ substitution. The solid solutions show low temperature independent resistance across the series, with small changes in magnitude that scale with the amount of substitution while displaying Pauli paramagnetic behavior throughout the solid solution. A novel series of quaternary pyrochlores with a general stoichiometry of BiCaMTeO7 (M = Cr, Ga, Sc, In, Fe) were synthesized and characterized for their structural, magnetic, and dielectric properties. A previously known analog BiCdFeTeO7 was structurally characterized for the first time. The cubic lattice parameter a shows an expected linear correlation with the average M site ionic radius for all the pyrochlore systems studied. Two representative compounds, BiCaFeTeO7 and BiCdFeTeO7, are structurally characterized utilizing high resolution synchrotron X-ray powder diffraction data revealing anion deficient pyrochlore systems which are off from the expected stoichiometry in respect to the M site. The A site of both pyrochlores are found to be moved off-center from the expected 16d site to the 96h displaced position with a magnitude of 0.25 Å and 0.22 Å for the Bi/Ca and Bi/Cd systems, respectively. The dielectric constants for the novel Bi/Ca system were found to be relatively high with a low dielectric loss and primarily independent of frequency and temperature. The magnetic measurements in the Bi/Ca system for the magnetic substitutions reveal a paramagnet and an antiferromagnet for the Fe and Cr analogs, respectively. This novel quaternary pyrochlore system shows great promise as an emerging dielectric material. A solid solution of Pr2-xCaxIr2O7 was synthesized and characterized for the structural, electrical and magnetic properties. Samples were prepared using standard solid state synthesis and resulted in metallic materials across the solid solution. High resolution synchrotron X-ray and time-of-flight powder diffraction data were utilized to perform detailed structural work and verify the composition of the pyrochlore iridate. The structural investigation reveals that the material crystallizes in the ideal pyrochlore structure with a space group of ̅3, while the lattice constants decrease in size indicating a complicated crystal chemistry for this solid solution. The structural, magnetic and electrical measurements provide evidence towards the systematic tuning of the Ir oxidation state with increasing Ca content

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