Reorientational Motion in Alkali-Metal Borohydrides: NMR Data for RbBH4 and CsBH4 and Systematics of the Activation Energy Variations

To study the reorientational motion of the BH4 groups in RbBH4 and CsBH4, we have performed nuclear magnetic resonance (NMR) measurements of the H-1 and B-11 spin-lattice relaxation rates in these compounds over wide ranges of temperature (48-400 K) and resonance frequency (14-90 MHz). It is found that at low temperatures the reorientational motion in CsBH4 is the fastest among all of the borohydrides studied so far. The activation energies E-a for BH4 reorientations obtained from our data are 138 +/- 4 meV for RbBH4 and 105 +/- 7 meV for CsBH4. Investigating the systematics of E-a variations in alkali-metal borohydrides MBH4, we have found a correlation between E-a and the relative deviation of the actual M-B distance from the sum of the ionic radii of M and BH4

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Iodide substitution in lithium borohydride, LiBH(4)-LiI

The new concept, anion substitution, is explored for possible improvement of hydrogen storage properties in the system LiBH(4)-LiI. The structural chemistry and the substitution mechanism are analyzed using Rietveld refinement of in situ synchrotron radiation powder X-ray diffraction (SR-PXD) data, attenuated total reflectance infrared spectroscopy (ATR-IR), differential scanning calorimetry (DSC) and Sieverts measurements. Anion substitution is observed as formation of two solid solutions of Li(BH(4))(1-x)I(x), which merge into one upon heating. The solid solutions have hexagonal structures (space group P6(3)mc) similar to the structures of h-LiBH(4) and beta-LiI. The solid solutions have iodide contents in the range similar to 0-62 mol% and are stable from below room temperature to the melting point at 330 degrees C. Thus the stability of the solid solutions is higher as compared to that of the orthorhombic and hexagonal polymorphs of LiBH(4) and alpha- and beta-LiI. Furthermore, the rehydrogenation properties of the iodide substituted solid solution Li(BH(4))(1-x)I(x), measured by the Sieverts method, are improved as compared to those of LiBH4. After four cycles of hydrogen release and uptake the Li(BH(4))(1-x)I(x) solid solution maintains 68% of the calculated hydrogen storage capacity in contrast to LiBH4, which maintains only 25% of the storage capacity after two cycles under identical conditions. (C) 2011 Elsevier B. V. All rights reserved

Impurity gas analysis of the decomposition of complex hydrides

This study aims at an investigation of the impurity gases emitted during the decomposition of borohydrides. For this we have set up a quantitative gas analysis based on a combination of FTIR spectroscopy and gravimetry. We show that the emission of various intermediates, in particular diborane, depends sensitively on the reaction conditions, including gas mean free path lengths, hydrogen backpressure, and sample pretreatment. Adduct-free Mg(BH 4)2 and LiBH4 emit diborane only at the impurity level, while for LiZn2(BH4)5 diborane is the main decomposition product. The decomposition reaction of LiZn 2(BH4)5 proceeds via a collision-induced dissociation of Zn(BH4)2 in Ar at ambient pressures. Various additives were tested aiming at catalyzing the decomposition of the desorbed diborane. © 2011 American Chemical Society