Halide Substitution in Magnesium Borohydride
journal contributionposted on 14.06.2012, 00:00 by Satoshi Hino, Jon Erling Fonneløp, Marta Corno, Olena Zavorotynska, Alessandro Damin, Bo Richter, Marcello Baricco, Torben R. Jensen, Magnus H. Sørby, Bjørn C. Hauback
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 β-Mg(BH4)2 in milled and annealed mixtures of Mg(BH4)2 with MgCl2/MgBr2 is smaller than that of pure β-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 α-Mg(BH4)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 MgCl2 and Mg(BH4)2, both in α- and β-polymorphs.