Pressure-dependent Deuterium Reaction Pathways in the Li-N-D System

Abstract

Neutron diffraction data from in situ deuteration of Li3N are presented under different pressure regimes, whereby reaction pathways differing from a two-stage (nitride- imide-amide) process are observed. At sufficiently high pressures, where the deuterium chemical potential is comparable with the heat of amide formation, the reaction appears to be driven straight to the amide plus deuteride phase mixture. At lower pressures, cubic phases exhibiting a concentration-dependent variation in lattice parameter are observed, interpreted here in terms of non-stoichiometric compounds covering two compositional ranges, with Li:N above and below that of the stoichiometric imide, Li2NH. It is argued that compounds with Li:N < 2 are compositionally intermediate between Li2NH and LiNH2, whilst those with Li:N > 2 are intermediate between Li4NH and Li2NH. The structural similarity between the cubic Li2NH phase and the tetragonal phases of Li4NH and LiNH2 is proposed to be a key factor in enabling a continuous variation in lattice parameter.