Preparation and crystal structure of ordered and disordered lithium nitride dichloride, Li5NCl2

A previously unknown ordered low-temperature phase Li5NCl2-II has been obtained from the disordered high-temperature phase Li5NCl2-I by slow cooling (100 degrees/h), while prolonged annealing at 400 degrees C resulted in a decomposition to Li4NCl and LiCl. Neutron powder diffraction data were collected for both compounds and analyzed by the Rietveld method of profile refinement. Li5NCl2-II crystallizes in the hexagonal rhombohedral space group R (3) over bar m, a = 366.10(3), c = 2851.6(3) pm with three formula units per unit cell. Its structure may be regarded as composed of lithium nitride-like [Li4N](+) layers and rock-salt-like [LiCl2](-) double-layers, stacked along the hexagonal c axis. A redetermination of the Li2O-type structure of Li5NCl2-I indicates the presence of N-Cl short-range order. (C) 1997 Academic Press.