Theoretical prediction of low-energy crystal structures and hydrogen storage energetics in Li2NH

We illustrate a systematic search method for determining hydrogen atomic arrangements in the hydrogen storage material, Li2NH. Our method relies on total-energy density-functional calculations, and yields a minimum-energy crystal structure for Li2NH as well as several low-energy metastable structures. Linear-response calculated Born effective charges show strong ionic interactions between the Li and N-H dimers, while the bonding between the N and H has covalent character. Including vibrational contributions, our orthorhombic Pnma structure yields a hydrogen storage Li2NH/LiNH2 reaction enthalpy of 63.7 kJ/mol H-2 at T=0 K, and 74.8 kJ/mol H-2 at T=300 K, in good agreement with experimental reports of similar to 66 kJ/mol H-2 for this reaction.