Date of Award
Doctor of Philosophy (PhD)
Dr. Joseph W. Kolis
Magnetically frustrated materials hold promise of unique behavior allowing for the novel study of quantum phenomena. Such materials are poised to become an integral foundation for technological advancement in the post-Silicon Age. Crystalline materials are given special focus where the rigid lattice allows more detailed study of these quantized effects and frustration behavior. As opposed to polycrystalline powders, large single crystals can be preferentially aligned enabling the study of anisotropic behavior. Two cubic structure types have garnered significant interest due to their 3-D tetrahedral arrangement of symmetry-related metal centers with the potential for magnetic frustration: pyrochlores and perovskites.
The supercritical hydrothermal crystal growth technique has been applied to a host of refractory oxides to enhance phase purity and minimize crystalline defects often introduced by conventional flux or melt-based based techniques. The supercritical growth in aqueous alkali at temperatures much lower than those of floating zone melts, while maintaining a sealed environment, avoids the possibility for reactant sublimation. In comparison to melt-based techniques, the modest thermal conditions also aid in minimizing atomic site displacements by insertion into different sites than those expected (“stuffing”). Through supercritical hydrothermal conditions, it was possible to obtain the entire series of lanthanide stannate pyrochlores. Stannous oxide, originally applied for in situ ceria reduction, was found beneficial for all lanthanide oxide reactants regardless of oxidation state allowing facile growth of faceted, mm-scale stannate octahedra in only days. This has allowed for the first single-crystal neutron scattering study of the quantum spin liquid candidate Ce2Sn2O7 with similar experiments to follow across the lanthanide series.
The sealed, pressurized supercritical hydrothermal technique was also applied in the growth of stoichiometric orthoscandates and site-ordered cubic barium double perovskites. Both systems had structures occur in crystallographic settings previously only possible through thin-film epitaxial growth or application of high pressures and temperatures. Attempts to expand the palette of lanthanide-containing germanium perovskites yielded novel P212121 LnTM(GeO4)(OH) (TM = 3d transition metal) adelite-type structures possessing chiral [TMO4]∞ a-axis chains. These adelites demonstrated the first known inclusion of lanthanides and germanium in the adelite-descloizite supergroup. The synthetic possibilities of the refractory lanthanide oxides and often-overlooked germanium offer great synthetic possibilities poised to further expand the adelite family while also providing a few magnetically frustrated surprises themselves or through the high-symmetry cubic garnet and spinel products encountered during exploratory adelite synthesis under hydrothermal conditions.
Powell, Matthew S., "Hydrothermal Synthesis of Frustrated Lanthanide Pyrochlores and Transition Metal Double Perovskites and Germanates" (2023). All Dissertations. 3281.