Phase Stability of Mixed-Cation Alkaline-Earth Hexaborides

We present the behavior of multiple solid solutions within ternary (Ba x Ca 1– x )B 6 and (Ba x Sr 1– x )B 6 compounds and demonstrate that nanodomain formation is preferred over uniform solid solutions under certain processing conditions. Instead of the expected single solid solution of M 1 and/or M 2 atoms within the MB 6 phase, we note separation into nanodomain regions rich in either M 1 or M 2 . This phase separation has been observed from detailed analyses of the shapes of the peaks in X-ray diffraction data, where peak splitting and asymmetry are the result of multiple solid solutions with lattice parameters differing by up to 1.4%. High-resolution transmission electron microscopy confirms the presence of these nanodomains, which are about 2–3 nm in size, and reveals varying degrees of lattice misalignment. We also present X-ray diffraction analysis of (Ba x Ca 1– x )B 6 powders calcined from 1273 to 1973 K and document the enhancement in sample homogeneity as the separated phases merge into a uniform solid solution. As subsequent calcinations at lower temperatures do not result in a re-separation of phases, the nanodomains are deemed metastable. The greatest degree of phase separation is observed in the (Ba x Ca 1– x )B 6 system, which corresponds to the largest difference in cation radii (0.161 vs. 0.134 nm for Ba 2+ and Ca 2+, respectively). Analysis of the chemical reactions that occur during synthesis suggests that the decomposition of the metal precursors (nitrates and carbonates) to metal oxides may cause selective MB 6 phase formation in mixed-cation hexaborides.