Mögliche Kristallstrukturen für oktaedrische MoleküleMX6bei dichtester Packung derX-Atome

Abstract

As long as there is no thermally induced disorder, octahedral MX6 molecules crystallize with a close-packed arrangement of X atoms in which one sixth of the octahedral holes are occupied by M atoms. On consideration of the symmetry restrictions imposed by the partial occupation of the octahedral holes, the possible space groups and structures are deduced with the aid of group--subgroup relationships. Centrosymmetric space groups that can be achieved include: (1) all trigonal, rhombohedral and monoclinic space groups, Pnmn, Pnma, Pnab and P{\bar 1} for hexagonal close packing of the X atoms; (2) Pcmn and Pcab for the double-hexagonal close packing; (3) Fddd, Bbmb, all monoclinic space groups and P{\bar 1} for cubic close packing. The most important molecular arrangements are illustrated and their expected cell dimensions are given. The known structures of WCl6, UCl6 and numerous hexafluorides correspond to some of the predicted possibilities.