Abstract
A hitherto unknown Li-ordered variant Li 6NBr 3I′ has been obtained from Li-disordered Li 6NBr 31 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 3I′ 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 LiN bondlength of 192.7 pm. The distortion from O h symmetry manifests in one LiNLi bondangle being significantly smaller than 90°, most likely arising from packing effects. The structure of Li-disordered Li 6NBr 3I has been redetermined. The improved Li distribution obtained is shown to be in agreement with orientationally disordered NLi 6 octahedra.
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