The discovery of reversible hydrogen uptake and release from sodium alanate (NaAlH4) doped with titanium by Bogdanovic and Schwickardi [1] in 1997, brought about a considerable increase in research activities in the field of hydrogen storage in complex hydrides. Many of the fundamental properties of complex hydrides had not been adequately investigated up that point, and a great number of important ques­tions remain to be answered to this date. Neutron scattering investigations should be an indispensable tool in this effort because of their great sensitivity to hydrogen. The interest in complex hydrides mainly stems from the high hydrogen content of light complex hydrides (e. g. 18.4 wt.% H2 in LiBH4 [2, 3]), which make them attractive as potential solid-state hydrogen storage materials. Complex hydrides may simply be viewed as salt-like compounds of the type An+[XHM]n where A is either an alkaline, alkaline earth, or early transition metal and X typically is B, Al, or N. The bond between hydrogen and X has covalent character and the resulting complex is a rather stable entity which fulfills the “18 electron” rule, i. e. the hydrogen containing entity such as [AlH4] has a closed-shell electronic structure. While complex hydrides do have high hydrogen-content, only a few systems exhibit reversible hydrogen uptake and release, and the thermodynamic and kinetic factors which govern the hydrogen exchange reaction are not yet fully understood. Hydrogen release and uptake is typically accompanied by a solid-state reaction which involves long-range diffusion of atoms heavier than hydrogen, which potentially limits fast reaction-rates. The covalent bond of hydrogen with the transition metal also appears to be very stable, so that the mechanisms for bond splitting (and reformation during hydrogen charging) still need to be fully understood.

Neutron scattering experiments can address virtually all fundamental atomic — level questions on the function of complex hydrides, such as the crystallographic and dynamic properties of compounds which are often not well characterized, materials development such as the mapping of the reaction pathway during hydrogen exchange, or technology development such as using imaging techniques to monitor the H uptake in prototype tanks. The focus of most investigations has been on structure determination using neutron powder diffraction and character­ization of the H dynamics using vibrational spectroscopy and quasielastic neutron scattering (QENS). The latter process gives access to stochastic motions of H, i. e. translational and rotational diffusion inside the crystal lattice, while vibrational spectroscopy can be used to determine the H density of states and has become an essential tool to benchmark first-principles calculations. In the following sections we will describe a few examples of neutron scattering experiments for the char­acterization of complex hydrides and their development as solid-state H storage materials. Apart from fundamental insights into these materials, which in many cases lack adequate characterization, phase transitions during hydrogen absorption and desorption can be monitored and thus giving insight into the reaction mecha­nism, intermediate reaction steps, and kinetic limitations.