The superstructure of KCuCe₂Se₆ due to ordering of copper cations

The quaternary compound KCuCe₂Se₆ was obtained as black needlelike crystals by reacting a mixture of K₂Se, Cu, Ce and Se in a 2:1:1:8 ratio at 450°C. The compound crystallizes in the orthorhombic space group Fddd with a = 8.5907(5), b = 11.3962(8) and c = 44.490(3) Å which is a superstructure of the already published structure of KCuCe₂Se₆ which was described in space group Immm. The Ce³⁺ ions are in bicapped distorted trigonal prismatic coordination of Se atoms and the Cu⁺ ions are in tetrahedral environment. The prisms are connected via triangular faces forming rods parallel to the crystallographic a axis. These rods are joined by one capping Se atom and one Se center of the triangular faces thus leading to the formation of anionic layers [CuCe₂Se₆]_n^n- within the ab plane. The Cu⁺ ions occupy only one half of the tetrahedral sites in an ordered fashion in contrast to the statistical occupation reported for the substructure. The charge balancing K⁺ ions are located between the anionic layers.