Darstellung und Lithiumteilstruktur von Lithiumnitridtribromid, Li6NBr3 und Lithiumnitridtriiodid, Li6NI3 / Preparation and Lithium Sublattice of Lithium Nitride Trihalides, Li6NHal3 (Hal = Br, I)

Abstract

Abstract Single phase Li6NBr3 was prepared by the reaction of Li3N and dry, OH-free LiBr at 450°C. It is the most bromide rich compound in the quasi binary system Li3-2xN1-xBrx. The crystal structures of Li6NBr3 and of isotypic Li6NI3, have been redetermined from neutron powder diffraction data. Both compounds crystallize in the cubic space group Fm3m (No. 225), a(Li6NBr3) = 894.18(7), a(Li6NI3) = 951.7(1) pm with 4 formula units per unit cell. The anion sublattice comprises a rock salt like arrangement of N with one of the three halogen atoms (Hal1), with the remaining halogen atoms in the resulting N4Hal4 cubes. The structure may be regarded as an ordered AX3 variation of a body-centered packing. The lithium atoms are disordered over the 96 (distorted) tetrahedral NX3 holes in such a way that of the four neighbouring, face-sharing NBr3 tetrahedra along an N-H all-connection line only one is occupied. The resulting Li coordination polyhedron around N (and Hal1) is a distorted octahedron with 6 equidistant Li atoms. Hal2 is surrounded by 12 Li atoms. The Li6NHal3 structure may be regarded as an anti-form of the cryolite structure.