In a series of in situ neutron diffraction measurements from deuterium up-take in Li3N, the present authors have identified a novel non-stoichiometric 'quasi-imide' of composition Li2-yNH, although the exact structure remains elusive, partly due to the presence of other compounds in reaction. A similar phase can be synthesised in a solid-state reaction from the reaction (1-y)Li4NH + yLi2NH, offering the possibility of studying the non-stoichiometric phase in isolation. The major objective of the proposed work, therefore, is an accurate structural determination of the Li2-yND phase. Identification of the H/D positions is key to this aim, thus neutron diffraction is essential to this end. An understanding of this phase will greatly facilitate the interpretation of the potential hydrogen storage compounds in the Li(Mg)-N-H system, currently being intensively researched worldwide.