<i>Ab initio</i>study of the low-temperature phases of lithium imide

We present a low-temperature structural model for lithium imide $({\text{Li}}_{2}\text{NH})$ that is consistent with experimental studies. Using the cluster expansion formalism and density-functional theory, we have identified a low-energy crystal structure for lithium imide with 96 atoms per unit cell. This low-energy structure is consistent with experimental diffraction patterns, and we propose that the symmetry of the structure may be increased at finite temperature due to thermal fluctuations. In addition, our results suggest that lithium motion is relatively facile between octahedral and tetrahedral sites, which may help explain how lithium diffuses through this material.