Doctoral Dissertations

Date of Award

8-2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Nuclear Engineering

Major Professor

Maik K. Lang

Committee Members

Haidong Zhou, Joshua R. Sangoro, Brian D. Wirth

Abstract

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.

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