Characterizing Local Order and Physical Properties of Rare Earth Complex Oxides

With more than 500 compositions, materials possessing the pyrochlore structure have a myriad of technological applications and physical phenomena. Three of the most noteworthy properties are the structure’s ability to resist amorphization making it a possible host matrix for spent nuclear fuel, its exotic magnetic properties arising from geometric frustration, and fast ionic conductivity for solid-oxide fuel cell applications. This work focuses on these three aspects of the pyrochlore’s many potential uses. Structural characterization revealed that pyrochlore-type oxides have a tendency to disorder from a high symmetry cubic structure to a lower symmetry orthorhombic arrangement in response to a variety of experimental conditions (i.e. changing composition, altering stoichiometry, and high-energy ion irradiation). The magnetic properties and structure of orthorhombic Dy₂TiO₅ [dysprosium titanate] have been successfully determined using neutron diffraction. Most notably, Dy₂TiO₅ displays a transition from two-dimensional to three-dimensional magnetic order at temperatures less than 2K and has magnetic moments that can order/disorder independently for each Dy site in response to an applied magnetic field. Broadband dielectric spectroscopy measurements also revealed that ionic conductivity in radiation-induced amorphous Gd₂Ti₂O₇ [gadolinium titanate] is more than 250 times larger compared with the crystalline phase. This dramatic increase is caused by a higher concentration of charge carriers coupled with enhanced mobility.