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

Abstract

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 Li 4 Mg(NH) 3, Li 2 Mg(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 Li 2 NH, we find a new Li 2 Mg(NH) 2 structure of Pca 2 1 symmetry, which has a lower T = 0 K energy than the previously reported Iba 2 phase. For the mixed imide Li 4 Mg(NH) 3, enumeration finds a structure belonging to the Imm 2 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 + Li 2 NH↔ Li 3 N + H 2 reaction; this step involves the formation of Li 4 NH according to 2LiH + Li 2 NH↔ Li 4 NH + H 2 . Additional novel mixtures of reactants with high gravimetric hydrogen storage densities are found using the calculated Li−Mg−N−H phase diagrams.