Structural and electronic properties of the hydrogen storage compound Ca(BH4)2·2NH3 from first-principles

Ca(BH4)2·2NH3 is considered to be a promising candidate for hydrogen storage. First-principles calculations based on density functional theory (DFT) were performed to study the structural and electronic properties. The optimized crystal structure was determined to be an orthorhombic Pbcn structure, with all atomic positions fully relaxed. The corresponding densities of states and charge densities indicated that strong correlations occur between Ca–B, B–H and N–H, but not between B–N, which obviously excluded the combination of calcium hydride and ammonia borane as the alternative structure. The presence of partial N–H⋯H–B dihydrogen bonding was verified. The calculated band structures implied an indirect wide band gap of 5.85 eV. The Bader charge analysis and calculated hydrogen removal energies were further investigated to explain the improved dehydrogenation properties of Ca(BH4)2·2NH3 compared to pristine Ca(BH4)2.