Time-of-flight neutron diffraction study on the Li 2O type phases of Li 6NBr 3

Abstract

A hitherto unknown low temperature phase Li 6NBr 3-II has been obtained from ordinary Li 6NBr 3 with anti-cryolite structure by slow cooling (20°/h) from 25°C to −130°C. Metastable at room temperature this new phase reversibly transforms to a further hitherto unknown phase, Li 6NBr 3-II′, at 100°C. A neutron diffraction study on Li 6NBr 3 phase II and II′ has been performed using the high resolution powder diffractometer (HRPD) at the spallation source ISIS, England. The structures of II and II′ are reported. Li 6NBr 3-II crystallizes in the space group I bca (No. 73) a=1150.047(4), b=1158.345(3), c=2249.777(7) pm with 16 formula units per unit cell. N and Br are found to form a TiFe 3 type 1:3 ordering variety of a cubic close packing. Li atoms occupy 3 4 of the tetrahedral voids in such a way that N and Br, which are both six coordinated by Li, comprise the same types of coordination polyhedra as do the metal atoms within the C-M 2O 3 (O 6M′Mω) structure. The distorted octahedral NLi 6 units with close to c 31 symmetry show a mean LiN bondlength of 195.0 pm and mean LiNLi bond angles of 84.7° and 95.3°, respectively. The less regular BrLi 6 groups are characterized by two short LiBr distances of about 254 pm beside four longer distances. Li 6NBr 3-II′ crystallizes in the space group I4/ mmm, a=584.414(2), c=1108.937(7) pm with two formula units per unit cell. It appears to be a lithium-disorder variety of Li 6NBr 3-II.