Time-of-flight neutron diffraction study on the cryolite type phases of Li₆NBr₃

A hitherto unknown Li-ordered variant Li₆NBr₃-I' has been obtained from Li-disordered Li₆NBr3-I 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. Li₆NBr₃-I' crystallizes in the space group P2₁/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, Na₃AlF₆. The characteristic structural unit is a distorted NLi₆ 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 Li₆NBr₃-I has been redetermined. The improved Li distribution obtained is shown to be in agreement with orientationally disordered NLi₆ octahedra.