Investigating Metastable Phases in Ion-Irradiated Binary Oxides

Metastable phases of chemical compounds often provide improved mechanical, thermal, or transport properties with respect to the equilibrium phases that exist under ambient conditions, hence the ubiquitous use of metastable materials in energy technologies. Using far-from-equilibrium processing, primarily ion irradiation experiments, the metastable phases in various binary oxides were produced and characterized. Analysis of irradiated A2O3 and AO2 materials yields information on ion-induced damage, phase transformations, and phase stability in this class of materials. In addition, metastable tetragonal ZrO2 was used as a model system to better understand the nature of structural metastability itself. Ion-irradiated and nanocrystalline ZrO2 were fully characterized across all length scales using advanced characterization techniques, including high-resolution synchrotron X-ray and spallation neutron total scattering. Pair distribution function analysis was used to solve the local structure and develop a model which (i) explains the relation between the atomic-scale structure and long-range periodicity of metastable materials, (ii) describes how metastable materials can be stabilized by incorporation of defects, and (iii) enables predictive capabilities to aid the discovery of new metastable phases. This research cultivates an improved understanding of the nature of metastability in crystalline structures, culminating in a prescribed approach to identifying new structural phases in metastable materials. The results of this project will help to improve existing structural models and more precisely engineer material properties across a wide breadth of materials used in nuclear technologies and beyond.