The crystal structures of K[NbC16]·NbC15 and NO[NbC16]·NbC1_5

摘要

The NbCI5 adducts of M[NbC16]·NbCI5 withM = K or NO are structurally related via their commonsupergroup Fmmm. K[NbC16] NbC15 crystallizes in thetriclinic space group Cl with a = 9.898(1), b = 9.443(1),c = 17.370(1) A, a = 89.57(1), = 108.49(1), y =90.54(1)° and Z = 4 at room temperature. For the mono-clinic nitrosonium compound with space group C2/c X-raydiffraction data have been recorded at 173 K givinga = 9.530(1), b = 9.412(1), c = 33.38(1) A, fi = 92.68(1)°and Z = 8. Layers of NbC15 dimers alternate with cation-centred doublelayers of the NbC16- ions, oriented parallel to the a-b plane.Compared to the cubic perovskite SrTiO3, the NbC16- 'oc-tahedra' of the K+ salt are uniformly rotated by 5° aroundthe stacking axis. On the contrary, the NO+ salt shows 'anti-ferrorotative' ordering by ±12° in alternate layers. Crystalsymmetry and twinning laws reflect the structural differ-ences, but the similarity between both structures prevails.