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The synthesis of halide-substituted Mg(BH 4) 2 by ball-milling, and characterization with respect to thermodynamics and crystal structure, has been addressed. The ball-milled mixture of Mg(BH 4) 2 and MgX 2 (X = Cl, Br) has been investigated by in situ/ex situ synchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), and infrared and Raman spectroscopy. High resolution SR-PXD patterns reveal that the unit cell volume of β-Mg(BH 4) 2 in milled and annealed mixtures of Mg(BH 4) 2 with MgCl 2 /MgBr 2 is smaller than that of pure β-Mg(BH 4) 2. This is due to substitution of BH 4 − by Cl − /Br − ions which have ionic radii smaller than that of BH 4 −. For comparison, ab initio calculations were run to simulate Cl substitution in α-Mg(BH 4) 2. The α-polymorph was used rather than the β-polymorph because the size of the unit cell was more manageable. Electronic energy data and thermodynamic considerations confirm the miscibility of MgCl 2 and Mg(BH 4) 2 , both in αand βpolymorphs.

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