Preparation and crystal structure of lithium nitride chloride Li4NCl

Li4NCl was prepared from Li3N and dry OH-free LiCl at 450 degrees C. It is found to be the more nitrogen-rich of two compounds in the quasi-binary system Li3-2xN1-xClx. Following unit cell indexing using laboratory X-ray powder data, the previously unknown structure of the title compound was solved from neutron powder diffraction data recorded using the flat-cone and powder diffractometer E2 at the Berlin BERII reactor. Li4NCl crystallizes in the hexagonal rhombohedral space group R (3) over bar m (No. 166), a = 366.225(4), c = 1977.18(4) pm with three formula units per unit cell. Its structure comprises a 1:1-ordering variant of a cubic close anion packing, N3- and Cl- forming layers perpendicular to [001]. Li atoms occupy all the triangular voids of the N layers and all the NCl3-tetrahedral holes, Nitrogen is in lithium nitride-like hexagonal bipyramidal coordination by Li, the equatorial six-membered ring being slightly puckered, while Cl is in distorted octahedral coordination. Calculation of the Madelung part of the lattice energy shows that the nitrogen hexagonal bipyramidal coordination polyhedron, peculiar for Li4NCl and the Li3N parent structure, is in agreement with a simple ionic picture and does not imply any covalent bonding. (C) 1997 Academic Press.