First-principles determination of multicomponent hydride phase diagrams: Application to the Li-Mg-N-H system

The first-principles calculations of total energies and vibrational free energies is used to define the multicomponent phase composition that maximize the hydrogen storage capacity in a given temperature and pressure. The method is applied to investigate the Li-Mg-N-H system for its suitability as a hydrogen storage material. The phase diagrams are determined by using grand-canonical Gibbs free energy for a system in contact with a gas-phase reservoir of hydrogen. The composition-temperature calculation results show that Li-Mg-N-H system exhibits stable compounds at temperatures below 130 K. The results also indicate that as a function of temperature, seven distinct reactions in the Li-Mg-N-H system exist, which involve a release of H₂. The Li ₂Mg(NH)₂ phase is found to be very stable and very high temperatures are required to decompose this compound.