Effective interactions between the N-H bond orientations in lithium imide and a proposed ground-state structure

Lithium imide (Li2NH) is a candidate material for hydrogen storage but its structure has proven difficult to fully characterize. The key problem is to identify the location of the hydrogen positions and N-H bond orientations in the structure. By searching an effective Hamiltonian in which the energy of lithium imide is expressed as a function of imide group orientations it is shown that nearest-neighbor imide groups prefer to orient antiparallel within a {110} plane of the antifluorite host structure. Although this orientation is frustrated in the antifluorite structure we suggest a ground state for Li2NH which has the lowest energy of any known structure as calculated by density functional theory.