Local disorder in lithium imide from density functional simulation and NMR spectroscopy

Abstract

Born–Oppenheimer molecular dynamics simulations in combination with calculations of 1H, 7Li, and 15N NMR chemical shifts are used to characterize lithium imide structures at different temperatures. Indications of the onset of local disorder in the lithium sublattice, leading eventually to superionicity, are recognized already at low temperature (100 K). Between 100 and 400 K, a new structure could be stabilized, which presents features that are intermediate between the previously reported Fm3m̅ and the Fd3m̅ structures. The disorder in the Li positions is associated with the reorientation of the NH bonds, which preferentially point toward Li-vacant sites. Clear signatures of such structural rearrangements are visible in the simulated NMR spectra, where smoother profiles are associated with a reduced amount of Li interstitials and a higher occupation probability of the antifluorite sites.