The crystal structure of K₂CuCl₃ and isomorphous substances

The crystal structure of K₂CuCl₃ and isomorphous substances has been determined. The crystals are orthorhombic with cell dimensions: a = 12.00, b = 12.55, c = 4.20 A. There are four molecules in the cell and the space group is Pnam. Weissenberg diagrams have been taken along the c axis with Cu K and Mo K radiation and the positions of the atoms have been fixed by means of an approximate three-dimensional Patterson analysis and a Fourier projection along the c axis. All atoms are situated in the symmetry planes perpendicular to the c axis. There are chains of the composition CuCl₃ in the direction of the c axis (needle axis) consisting of CuCl₄ tetrahedra sharing corners; they are more compact than the SiO₃ chains in the pyroxenes. The K atoms are situated between these chains, surrounded by seven Cl atoms. The direction of the chains is in agreement with the optical behaviour: largest refractive index in the needle direction.