Local Disorder in Hydrogen Storage Compounds: The Case of Lithium Amide/Imide

Amides and imides of alkali metals are a very promising class of materials for use as a hydrogen-storage system, as they are able to store and release hydrogen via a chemical route at controllable temperatures and pressures. We critically revise the present picture of the atomic structure of the lightest member (LiNH2/Li2NH) by using a combined computational and experimental approach. Specifically, ab initio path integral molecular dynamics simulations and solid-state H-1 NMR techniques are combined. The results show that the presently assumed local structure might be inconsistent or at least incomplete and needs considerable revision. In particular, the Li atoms turn out to be more mobile and more disordered than suggested by structural data obtained from X-ray scattering. Also, the configuration of the hydrogen atoms, which is accessible via the NMR experiment and the corresponding first-principles calculations, is different from the previously assumed data. The computed and experimentally observed H-1 NMR parameters are in very good mutual agreement and illustrate the unusual chemical environment of the hydrogen atoms in this system. Incorporating our results on the new lithium data, we show that the effect of nuclear quantum delocalization for the hydrogen atoms is considerably reduced compared to the perfect crystal structure.