Crystal structure and pseudosymmetry of a new modification of potassium hexachloroniobate(V), KNbCl6. Comments on the cubic phase

Long needles of KNbCl6 - invariably twinned around [100] - are obtained if the material is crystallized from SOCl2 solution. The structure has been determined from X-ray data collected with a single-crystal diffractometer at room temperature [space group P21/n, Z = 16, a = 6.894(1), b = 22.073(4), c 23.337(3) A, β = 91.00(1) , R = 0.032 for 2909 unique reflexions, 290 structural parameters]. Distorted NbCl6- octahedra and 'interstitial' K+ ions are found to form similar arrangements, each of them corresponding to a closest packing of spheres with the layer sequence ACAB (stacking symbol hc). The resulting asymmetry in coordination by potassium is coupled with a strong off-centre displacement of the Nb atoms in any of the four independent chlorine polyhedra (0.14 A on average). A pronounced pseudosymmetry accounts for the twinning. Since P21/m21/n21/b (no. 62) is already a good approximation of the real structure, only one formal step of symmetry reduction (index t2) is needed to create both, the observed twin law and the actual space group P1 21/n1. Above 180 C a reconstructive phase transition leads to the 'face-centred cubic' modification with ∝ 10% lower density. (orig.)