Halide substitution in magnesium borohydride

The synthesis of halide-substituted Mg(BH4)2 by ball-milling, and characterization with respect to thermodynamics and crystal structure, has been addressed. The ball-milled mixture of Mg(BH4)2 and MgX2 (X = Cl, Br) has been investigated by in situ/ex situ synchrotron powder X-ray di\ufb00raction (SR-PXD), di\ufb00erential scanning calorimetry (DSC), and infrared and Raman spectroscopy. High resolution SR-PXD patterns reveal that the unit cell volume of \u3b2-Mg(BH4)2 in milled and annealed mixtures of Mg(BH4)2 with MgCl2/MgBr2 is smaller than that of pure \u3b2-Mg(BH4)2. This is due to substitution of BH4 12 by Cl 12/Br 12 ions which have ionic radii smaller than that of BH4 12. For comparison, ab initio calculations were run to simulate Cl substitution in \u3b1-Mg(BH4)2. The \u3b1-polymorph was used rather than the \u3b2-polymorph because the size of the unit cell was more manageable. Electronic energy data and thermodynamic considerations con\ufb01rm the miscibility of MgCl2 and Mg(BH4)2, both in \u3b1- and \u3b2-polymorphs