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

Abstract

A hitherto unknown Li-ordered variant Li 6NBr 3I′ has been obtained from Li-disordered Li 6NBr 31 on cooling. I′ is stable only over a restricted temperature range from −60 to −80°. Its structure has been solved from neutron powder diffraction data collected using the high resolution powder diffractometer (HRPD) at the spallation source ISIS, UK. Li 6NBr 3I′ crystallizes in the space group P2 1/n (No. 14) a = 6.26585(4), b = 6.29839(4), c = 8.95577(6) pm, β = 90.171(1)° with two formula units per unit cell. Its structure is an antitype of that of the mineral cryolite, Na 3AlF 6. The characteristic structural unit is a distorted NLi 6 octahedron approximating D 2h symmetry with an average LiN bondlength of 192.7 pm. The distortion from O h symmetry manifests in one LiNLi bondangle being significantly smaller than 90°, most likely arising from packing effects. The structure of Li-disordered Li 6NBr 3I has been redetermined. The improved Li distribution obtained is shown to be in agreement with orientationally disordered NLi 6 octahedra.