The following work provides new insight into the behavior of the alkaline-earth hexaborides CaB6, SrB6 and BaB6. Both binary and ternary compounds were produced for the first time by solution combustion synthesis, resulting in cubic particles averaging 500 nm in size. Detailed X-ray diffraction analysis revealed phase separation and formation of nano-domains in ternary (Ba-Ca)B6 and (Ba-Sr)B6 compounds. High- resolution transmission electron microscopy confirms the presence of these phases in the form of homogenous nano-regimes in individual equilibrium within the hexaboride particles. Thermal treatments up to 2000 K enhance the overall homogeneity of the samples and merge the phases into one, indicating that the as-synthesized state is unstable as the collection of nano-regimes results in increased microstrain and lattice imperfections. Analysis of the chemical reactions that occur during synthesis suggests that the decomposition of the metal precursors to metal oxides introduces variance into the formation process of mixed-cation hexaboride compounds, producing nano-regimes within the crystal lattice. Spark plasma sintering was shown to effectively produce high- density hexaboride samples from powders in as little as 45 minutes. The effect of pulsed direct current on metal atom diffusion in CaB6-SrB6 couples was investigated. It was shown that the diffusivity of Ca and Sr atoms across the interface was towards the positive electrode when subjected to a current flow of 2.2 kA at a temperature of 2007 K. No new phases were formed at the interface between CaB6 and SrB6 as hexaboride compounds readily form solid solutions. Energy-dispersive spectroscopy was used to map the concentration of Ca and Sr surrounding the interface, and diffusion profiles were fitted with error functions. The concentration curves display concentration-dependent diffusivity. Total dopant values (Q) were used to numerically compare the differences between Ca diffusion in SrB6 and Sr diffusion in CaB6 under the influence of current. The results provide evidence for the role of pulsed direct current on the diffusion of metal ions in hexaboride compounds.

The concept of the species has traditionally been the principal unit for the classification of the diversity of life. Although the conceptual and legal relevance of the idea of species is profound and wide ranging it is also increasingly evident that the concept of species is a significant oversimplification of the complex processes at work in nature. The growing body of evidence from genomic data, and other sources, indicates that interspecific hybridization plays a much more substantial role in evolution than had previously been appreciated and that speciation, the formation of new species, is likely a more gradual process than had been appreciated. In my dissertation work, I have applied whole genome sequencing and ancient DNA based techniques to examine the evolutionary relationship of two closely related species, polar bears (Ursus maritimus) and brown bears (Ursus arctos), which occupy substantially different ecological niches. This work is presented in a series of three papers that have contributed to reshaping scientific understanding of the relatedness of polar bears and brown bears and suggest that climate changes can profoundly impact the frequency of hybridization between these two species. I have also contributed to method development – in a fourth paper my coauthors and I developed a novel application of existing analysis tools to estimate the end point of the speciation process, the time at which all gene flow has ceased, from genomic data. Finally, in collaboration with my co-first author Axel Barlow and colleagues, I have examined the evolutionary relationship between polar and brown bears and their closest extinct relatives, the cave bears, which has revealed previously undocumented admixture between cave bears and brown bears and provides insights into the extent of diversity within the paleo-species groups of cave bear.