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A Study on the Phase Stability and Diffusion Behavior of Alkaline-Earth Hexaborides


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.

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