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−xCl_x)₂ solid solutions indicate compositions from x = 0 to 0.6, depending on the amount of CaCl₂ in the parent mixtures. β-Ca((BH₄)_0.5Cl_0.5)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.