Halide substitution in Ca(BH4)2

Halide substitution in Ca(BH4)2 has been investigated in ball milled mixtures of Ca(BH4)2 and CaX2 (X ¼ 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 a- to b-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 b-Ca((BH4)1xClx)2 solid solutions indicate compositions from x ¼ 0 to 0.6, depending on the amount of CaCl2 in the parent mixtures. b-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.