Halide substitution in Ca(BH₄)₂

Abstract

Halide substitution in Ca(BH4)2 has been investigated in ball milled mixtures of Ca(BH4)2 and CaX2 (X = F, Cl, Br) with different molar ratios. In situ synchrotron radiation powder X-ray diffraction measurements of Ca(BH4)2 + CaCl2 with 1 : 0.5, 1 : 1 and 1 : 2 molar ratios reveal that no substitution of Cl− for BH4− occurs from the ball milling process. However, substitution readily occurs after the transitions from α- to β-Ca(BH4)2 and from orthorhombic to tetragonal CaCl2 upon heating above ∼250 °C, which is evident from both contraction of the unit cell and changes in the relative Bragg peak intensities, in agreement with theoretical calculations. Rietveld analyses of the obtained β-Ca((BH4)1−xClx)2 solid solutions indicate compositions from x = 0 to 0.6, depending on the amount of CaCl2 in the parent mixtures. β-Ca((BH4)0.5Cl0.5)2 was investigated by differential scanning calorimetry and has a slightly higher decomposition temperature compared to pure Ca(BH4)2. No substitution with CaF2 or CaBr2 is observed.