Synthesis and characterization of Magnesium-Iron-Cobalt complex hydrides

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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 \ubc 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 \ubc 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

Combined X-ray and Raman Studies on the Effect of Cobalt Additives on the Decomposition of Magnesium Borohydride

Magnesium borohydride (Mg(BH4)2) is one of the most promising hydrogen storage materials. Its kinetics of hydrogen desorption, reversibility, and complex reaction pathways during decomposition and rehydrogenation, however, present a challenge, which has been often addressed by using transition metal compounds as additives. In this work the decomposition of Mg(BH4)2 ball-milled with CoCl2 and CoF2 additives, was studied by means of a combination of several in-situ techniques. Synchrotron X-ray diffraction and Raman spectroscopy were used to follow the phase transitions and decomposition of Mg(BH4)2. By comparison with pure milled Mg(BH4)2, the temperature for the \u3b3 \u2192 \u3b5 phase transition in the samples with CoF2 or CoCl2 additives was reduced by 10\u201345 \ub0C. In-situ Raman measurements showed the formation of a decomposition phase with vibrations at 2513, 2411 and 766 cm 121 in the sample with CoF2. Simultaneous X-ray absorption measurements at the Co K-edge revealed that the additives chemically transformed to other species. CoF2 slowly reacted upon heating till ~290 \ub0C, whereas CoCl2 transformed drastically at ~180 \ub0C

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