Phase Formation in Divalent Hexaborides: Prospects for Hydrogen Storage Technologies


Hexaborides are a unique class of non-oxide ceramics with many interesting electronic, magnetic and optical properties. The cubic crystal structure consists of covalently bonded boron octahedra surrounding a loosely-bonded metal ion, which donates electrons to the boron framework and directly influences the compound’s conductivity. Compatible metal ions are restricted in size by the boron sublattice, and therefore readily form solid solutions in mixed-ion compounds. As with many borides, hexaborides have a high melting temperature and often require synthesis temperatures over 1000°C. A combustion technique presents a low-temperature (400°C) synthesis alternative and subsequently produces sub-micron particles. CaB6, SrB6, and BaB6 powders were produced in this fashion, along with binary mixtures of each in 10 mol% increments. X-ray diffraction was used to study the phases formed, and while the (Ca-Sr)B6 and (Ba-Sr)B6 systems form single solid solutions, the (Ba-Ca)B6 system separates into two solid solution phases. The figure depicts the lattice parameters for each of the two solid solutions versus Ba content. While the difference in ionic radii between Ba and Ca is still within the Hume-Rothery 15%, this phase separation suggests a clustering effect of the Ba and Ca ions. The X-ray diffraction data suggests high strains in the 50% Ba regimes; a possible driving force for segregation. (Ba-Ca)B6 powders were thinned for atomic resolution TEM to see if the clustering resulted in observable differences in the lattice.

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