LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.

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We have investigated the crystal structure of Ca(BD4)2 by combined synchrotron radiation X-ray powder diffraction, neutron powder diffraction, and ab initio calculations. Ca(BD4)2 shows a variety of structures depending on the synthesis and temperature of the samples. An unknown tetragonal crystal of Ca(BD4)2, the beta phase has been solved from diffraction data measured at 480 K on a sample synthesized by solid-gas mechanochemical reaction by using MgB2 as starting material. Above 400 K, this sample has the particularity to be almost completely into the beta phase of Ca(BD4)2. Seven tetragonal structure candidates gave similar fit of the experimental data. However, combined experimental and ab initio calculations have shown that the best description of the structure is with the space group P4(2)/m based on appropriate size/geometry of the (BD4)tetrahedra, the lowest calculated formation energy, and real positive vibrational energy, indicating a stable structure. At room temperature, this sample consists mainly of the previously reported alpha phase with space group Fddd. In the diffraction data, we have identified weak peaks of a hitherto unsolved structure of an orthorombic gamma phase of Ca(BD4)2. To properly fit the diffraction data used to solve and refine the structure of the beta phase, a preliminary structural model of the gamma phase was used. A second set of diffraction data on a sample synthesized by wet chemical method, where the gamma phase is present in significant amount, allowed us to index this phase and determine the preliminary model with space group Pbca. Ab initio calculations provide formation energies of the alpha phase and beta phase of the same order of magnitude (delta H < or = 0.15 eV). This indicates the possibility of coexistence of these phases at the same thermodynamical conditions.