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 diffraction (SR-PXD), differential scanning calorimetry (DSC), and infrared and Raman spectroscopy. High resolution SR-PXD patterns reveal that the unit cell volume of beta-Mg(BH4)(2) in milled and annealed mixtures of Mg(BH4)(2) with MgCl2/MgBr2 is smaller than that of pure,beta-Mg(BH4)(2). This is due to substitution of BH4- by Cl-/Br- ions which have ionic radii smaller than that of BH4-. For comparison, ab initio calculations were run to simulate Cl substitution in alpha-Mg(BH4)(2). The alpha-polymorph was used rather than the beta-polymorph because the size of the unit cell was more manageable. Electronic energy data and thermodynamic considerations confirm the miscibility of MgCl2 and Mg(BH4)(2), both in alpha- and beta-polymorphs.