The crystal structures of K[NbCl6] · NbCl5 and NO[NbCl6] · NbCl5

The NbCl 5 adducts of M[NbCl 6 ] · NbCl 5 with M = K or NO are structurally related via their common supergroup Fmmm. K[NbCl 6 ] · NbCl 5 crystallizes in the triclinic space group C1 with a = 9.898(1), b = 9.443(1), c = 17.370(1) A, α = 89.57(1), β = 108.49(1), γ = 90.54(1)° and Z = 4 at room temperature. For the monoclinic nitrosonium compound with space group C2/c X-ray diffraction data have been recorded at 173 K giving a = 9.530(1), b = 9.412(1), c = 33.38(1) A, β = 92.68(1)° and Z = 8. Layers of NbCl 5 dimers alternate with cation-centred double layers of the NbCl 6 - ions, oriented parallel to the a-b plane. Compared to the cubic perovskite SrTi0 3 , the NbCl 6 - 'octahedra' of the K + salt are uniformly rotated by 5° around the stacking axis. On the contrary, the NO + salt shows 'antiferrorotative' ordering by ±12° in alternate layers. Crystal symmetry and twinning laws reflect the structural differences, but the similarity between both structures prevails.