First-Principles Study of the Li−Mg−N−H System: Compound Structures and Hydrogen-Storage Properties

Crystal structures and hydrogen storage properties of compounds belonging to the Li−Mg−N−H system are studied using first-principles DFT calculations and the grand-canonical linear programming approach (Akbarzadeh, et al. Adv. Mater. 2007, 19, 3233). We report the static, zero-point, and T > 0 K vibrational energies of all known storage compounds belonging to this system along with new predictions for Li4Mg(NH)3, Li2Mg(NH)2, and MgNH. Enthalpies, entropies, and hydrogen release temperatures are calculated for all thermodynamically reversible dehydrogenation reactions occurring within 300 °C of room temperature. Using a structural enumeration approach based on the antifluorite lattice of Li2NH, we find a new Li2Mg(NH)2 structure of Pca21 symmetry, which has a lower T = 0 K energy than the previously reported Iba2 phase. For the mixed imide Li4Mg(NH)3, enumeration finds a structure belonging to the Imm2 space group with two formula units per conventional cell. It is also shown that a previously unsuspected intermediate step is thermodynamically favored in the well-known LiH + Li2NH↔ Li3N + H2 reaction; this step involves the formation of Li4NH according to 2LiH + Li2NH↔ Li4NH + H2. Additional novel mixtures of reactants with high gravimetric hydrogen storage densities are found using the calculated Li−Mg−N−H phase diagrams.