Borohydrides are also under investigation as potential hydrogen storage materials, particularly those with high hydrogen contents. For instance, LiBH4 can be reformed from the elements at 973 K and 150 bar H2 [31]. Conditions are slightly milder when LiBx precursors such as LiB3 or Li7B6 intermetallics are used. Other systems with limited reversibility include e. g. Mg(BH4)2 or Ca(BH4)2 which can be (partially) rehydrogenated at high pressures (90 MPa) and high temperatures (673 K) [32, 33]. Mg(BH4)2 would be an attractive storage material since it has a gravimetric storage capacity of 14.9 wt.% H2 and the reported enthalpy of reaction is 39 kJmol-1 for the reaction Mg(BH4)2 ^ MgH2 + B + 3H2 [34], so that the estimated equilibrium pressure at RT is 1 bar H2.

The structure of LiBH4 at low temperature is orthorhombic (LT) and changes near 380 K to a hexagonal high-temperature (HT) structure [35]. The four H atoms are bound covalently to the central B atom. Early structural studies found a strong asymmetry of the BH4-tetrahedron [36] in the LT phase, while later reports describe the geometry of the BH4-entities to be close to that of an ideal tetrahedron both below and above the structural phase transition [35]. The B-H distance was esti­mated to be dB-H = 1.16-1.26 A in the HT phase from synchrotron X-ray diffraction [35], while the B-D distances were determined to be dB-D = 1.18-1.20 A at 302 K in neutron diffraction experiments [37]. The structural phase transition is accompanied by increased thermal motion of the BH4-complexes, which is indicative of an order — disorder transition. The nature of the reorientations of the BH4-tetrahedron was investigated using quasielastic neutron scattering (QENS) for both the high tem­perature and low temperature phase. The type of reorientation was identified from the elastic incoherent structure-factor (EISF), which is proportional to the Fourier transform of the probability density of finding a hydrogen atom at a given position. The hydrogen mobility in both phases can be described by rotational jumps of the BH4-tetrahedra. The mean residence times т between successive jumps are on the order of a few picoseconds at temperatures near the phase transition [38]. While 120° rotations around one C3 symmetry axis of the tetrahedron could be identified in the LT phase [38], but there are also clear indications for two different dynamic processes with distinct activation energies both from NMR data [39], as well as from inelastic fixed window scans in QENS experiments [40]. The reported acti­vation energies are 162 ± 2 and 232 ± 11 meV, respectively. The BH4 dynamics becomes even faster above the phase transition [38] and the QENS data can then best be described by an orbit exchange-model [41] where three out of the four H atoms rotate fairly freely around a C3 axis while the axial H displays only occa­sional jumps as illustrated in Fig. 8.3.

The EISF obtained for HT LiBH4 may be compared (Fig. 8.3) with theoretical models for several reorientation mechanisms, such as 120° rotations around the C3 axis, tetragonal or cubic tumbling and the orbit exchange-model. It is apparent that it is essential to monitor as much of the reciprocal space as possible in the QENS exper­iments for the identification of the true nature of the motions, since a clear distinction

between different models can only be made above Q = 2.5 A 1. The best agreement between the prediction of a given reorientation model and the experimental data is obtained for the orbit exchange-model where frequent H jumps occur between sites on a circle around the C3 axis while the axial H only occasionally jumps (see Fig. 8.3, lines d and e). The model was calculated using either N = 6 or 12 equivalent positions on the circle, however already N =6 was sufficient to describe the data.

While reorientation of the BH4 — tetrahedra has been observed at temperatures as low as 175 K, translational diffusion of BH4 could only be seen above the melting transition [42] at 553 K. Exchange of atomic hydrogen between adjacent BH4 tetrahedra remains slow even at these temperatures and transport of H takes mainly place by way of the BH4- units. Li+ mobility, on the other hand, becomes much faster when going from the LT to the HT phase, which is indicated by a an increase of three orders of magnitude of the Li+ conductivity from 10-6 -10-8 Scm-1 in the LT phase to 10-3 Scm-1 in the HT phase [43]. Substitution of the BH4- units by I in LiBH4/LiI mixtures results in a stabilization of the HT phase at room temperature (RT). QENS measurements accordingly show significantly-enhanced dynamics of BH4 at RT compared to the LT phase (at RT) and a similar reorientation mechanism as in the pure LiBH4 HT phase [44]. The stabilization of the HT phase by I substitution also preserves the high Li+-conductivity at RT. It has been suggested that Li+ conductivity is supported by the BH4- movements [43] in a paddle-wheel like mechanism.

The alkaline borohydrides NaBH4 and KBH4 exhibit a structural phase-transi­tion associated with an order-disorder transition of the BH4 subunits, from a tetragonal LT phase to a cubic HT phase (at * 190 K for NaBH4 [45], and 65-70 K for KBH4 [46]). QENS data on NaBH4 suggest reorientational jumps around the C2 and C3 axis, which is in agreement with the crystal symmetry. Reorientation of the BH4 entities around a C4 axis have been reported for both NaBH4 and KBH4, so that the four hydrogens of the tetrahedron occupy the eight corners of a cube but with half occupancy [47]. Thermodynamic measurements provided evidence for an order-disorder transition in RbBH4 at 44 K and in CsBH4 at 27 K [48] but this could not be discerned from neutron powder diffraction down to low temperature [46]. However, vibrational spectroscopy of the torsion band indicates that the heavier alkali borohydrides also exhibit a similar order-disorder transition although the ordering of the BH4 seems to occur on a shorter length scale [47].

The variety of structural phases is especially rich for the alkaline earth borohy — drides Mg(BH4)2 and Ca(BH4)2. Both of these can be synthesized in solvent free form, but the crystal structure depends strongly on the preparation conditions. Two polymorphs were initially identified for Mg(BH4)2: an а-phase, which transforms irreversibly into P-Mg(BH4)2 at temperatures above 490 K. Both polymorphs have unexpectedly-complex crystal structures which differ from numerous theoretical predictions [49-52]. Crystal structure determination from powder data proved dif­ficult especially concerning the hydrogen positions but eventually, the structure of the а-phase was identified as hexagonal with space group P6122 [50]. Adjacent BH4- groups are coordinated by a central Mg via two opposite edges of the tetrahedron, but the resulting Mg-H2BH2-Mg bridges are not exactly planar. a-Mg(BH4)2 has 6.4 % unoccupied voids (amounting to 37 A2 per unit cell) whereas P-Mg(BH4)2 which has an orthorhombic structure (Fddd) and is much more dense with no unoccupied voids. The а and P-polymorphs are built up of a corner sharing network of Mg2+[BH4 ]4 tetrahedra. A highly porous form, y-Mg(BH4)2, was synthesized more recently, and found to possess a cubic structure (Id 3a) with a network of interconnected channels and 33 % porosity, which could be utilized for the adsorption of molecular hydrogen (or nitrogen) at low temperatures [52]. Applica­tion of pressure (1-1.6 GPa, diamond anvil cell) causes the structure to collapse and results in tetragonal 6-Mg(BH4)2 [52]. Synchrotron X-ray diffraction studies iden­tified yet another polymorph, e-Mg(BH4)2, during the decomposition y-Mg(BH4)2 [53]. DFT calculations find that many structural models for Mg(BH4)2 are nearly degenerate in energy, which can explain difficulties with the correct prediction of crystal structures and decomposition pathways [54, 55]. Moreover it is likely that more polymorphs of Mg(BH4)2 exist depending on preparation conditions (and/or impurity content).

The reorientational motion of the BH4-tetrahedra in P-Mg(BH4)2 have been studied using QENS on two instruments with different energy resolution and therefore different timescales. Two thermally-activated reorientation processes have been observed on these timescales in the temperature range from 120-437 K, and have been identified as rotations around the C2||-axis (which connects the two Mg atoms in the Mg-H2BH2-Mg bridge) or around the C3-axis of the tetrahedron [56] (Fig. 8.4).

Fig. 8.4 Left The measured and modelled EISFs for p-Mg(BH4)2: Using the mica analyser high resolution time-of-flight backscattering spectrometer (MARS) at the Paul Schemer Institute: (red plus) 120 K, (blue x) 150 K, (red triangle) 180 K, (green asterisk) 210 K, and (black hexagon) 240 K. Using the spectrometer for high energy resolution (SPHERES) at the Heinz Maier-Leibnitz Zentrum: (red square) 318 K, (green triangle) 365 K, and (blue circle) 473 K. The calculated EISF are for the reorientation models: (—) C2n (MARS data) and C3 (SPHERES data) rotational diffusion-model and hindered rotation diffusion-model (—). Right: BH4 reorientation as hindered rotation around C2||. Reprinted with permission from (P. Martelli, D. Blanchard, J. B. Maronsson, M. D. Riktor, J. Kheres, D. Sveinbj, E. G. Bardaji, J. Phys. Chem. C 116, 2013 (2013)) [56]. Copyright (2013) American Chemical Society

The activation energies were determined to be 39 ± 0.5 meV and 76 ± 5 meV for the C2|| rotation and 214 ± 4 meV for the C3 axis rotation, respectively [55]. Low — frequency vibrational spectroscopy using Raman scattering revealed a complex structure of both internal and external modes in a mixed a/p-Mg(BH4)2 sample which is considerably modified when passing through the phase transition [57]. Neutron spectroscopy revealed an additional band which was attributed to BH4 librations [56] but more spectroscopic work along with DFT calculations is needed for a more comprehensive understanding of Mg(BH4)2.

Ca(BH4)2 is another interesting candidate for storage application with a gravi­metric hydrogen storage density of 11.6 wt.% H2 or 130 kgm-3 of H2. There are four known polymorphs: a, a’, and у-phase, which are considered LT phases while P-Ca (BH4)2 is an HT phase. The a and у modification have an orthorhombic structure (Fddd [58, 59] or F2dd [60] for a-Ca(BHr)2 and Pbca [58] for y-Ca(BH4)2) whereas the a’-modification is tetragonal (142d [59]). Above 400 K, a tetragonal (P4) high temperature phase P-Ca(BH4)2 is observed [57, 58]. Each Ca is surrounded by 6 boron atoms in the a, P, and у-phases forming a CaB6 octahedron [58] and the polymorphism results from different arrangements and connections of the CaB6 octahedra. The energy differences of the different polymorphs are small [57, 61] which explains why frequently more than one crystalline phase is found in the same sample batch. Low energy spectroscopic measurements indicate that the phase stability is linked to the librations of the BH4- units [62] and it was suggested that entropic contributions are driving the phase transitions. Moreover, the different polymorphs exhibit different decomposition properties [63], hence a thorough understanding of the bare materials is essential for their potential application in solid-state hydrogen storage. The hydrogen dynamics in (predominantly) P-Ca(BH4)2 have been investigated by QENS [64]. Two different thermally-acti­vated rotational-reorientation processes have been identified in the temperature range from 100 to 260 K, which were attributed to rotations around the C2 and C3 axis, respectively. Translational diffusion with a jump length of 2.5 A was also observed, which the authors attributed to the H2 diffusion of trapped impurities, while no long-range diffusion of the BH4 units was observed at these temperatures.

Besides the pure alkaline and alkaline-earth metal borohydrides, mixtures combining different cations have been investigated [65-67] in an attempt to modify the reaction enthalpy for hydrogen desorption. The decomposition temperature for borohydrides correlates with the electronegativity of the cation [68, 69] and thus dual cation borohydrides could be ideal candidates for tuning the thermodynamic properties.