First-principles study on lithium amide for hydrogen storage

ne fundamental properties of lithium amide LiNH2, which is fully hydrogenated phase of lithium nitride Li3N, have been investigated by the first-principles calculations using the ultrasoft pseudopotential method, including the structural, electronic, dielectric, and vibrational properties. The calculated structural parameters agree well with the experimental data except for hydrogen positions. The analyses for the electronic structure and the Born effective charge tensors indicate an ionic feature between Li+ and [NH2](-). The internal bonding of [NH2](-) anions is primarily covalent. The internal N-H bending and stretching vibrations of [NH2](-) anions yield Gamma-phonon modes around 1500 and 3400 cm(-1), respectively. These can be fairly reproduced by the molecular approximation, suggesting a strong internal bonding of [NH2](-) anions. The heat of formation for the fully hydriding reaction of Li3N is predicted as -85 kJ/mol H-2 which agrees well with the experimental value. Some discussions are also presented for the properties of Li3N.