SPACE GROUP OF THE TRIRUTILE TYPE-STRUCTURE OF LI2MOF6

Dilithium molybdenum hexafluoride, Li2MoF6, tetragonal, M(r) = 223.83, a = 4.6863 (7), c = 9.191 (2) angstrom, V = 201.8 (2) angstrom3, D(x) = 3.683 Mg m-3, lambda(Mo Kalpha) = 0.71069 angstrom, F(000) = 206, s ingle crystal refinement to R = 0.057 in space group P4(2)/m2(1)n2/m b ased on 371 structure factors measured on a diffractometer by Brunton [(1971). Mater. Res. Bull. 6, 555] who preferred in his refinement of the same data space group P4(2)2(1)2. Refinements in space groups P4(2 ), P4(2)2(1)2 and P4(2)/m2(1)/n2/m show that Li(2)MoF6 crystallizes in the most highly symmetric of these space groups and is, therefore, is ostructural with the aristotypic trirutile type. The decision for spac e group P4(2)/m2(1)/n2/m is based on the shape and orientation of the displacement ellipsoid of the F(1) atom and on the extremely high R va lue of the reflections not obeying the extinction condition of the n g lide in space group P4(2)2(1)2. The importance of the weak reflections in deciding a centrosymmetric-non-centrosymmetric space-group ambigui ty is emphasized.