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Binary and ternary transition-metal chalcogenides have technological interest as they exhibit physical properties which make them usable, for example, for applications as infrared lenses ['', or color phosphors[3]. The usual way for preparing lanthanoid chalcogenides is the high-temperature route which often leads to mixtures of phases or compounds with defect structures. Another way is the metathesis between alkali chalcogenides and lanthanoid halides.Two known structure types in the alkali metal (A)/lanthanoid (Ln)lchalcogenide (Q) system have the composition ALnQ2r41. One of these structure types is a simple NaCl structure with statistically distributed A+ and Ln3+ cations. The other ALnQ2 compounds crystallize in the a-NaFe02 structure type. A third type is represented by the compounds KLnMQ4 (Ln = La, Nd, Gd, Y; M = Si, Ge; Q = S, Se)I51.A new method for preparing ternary and higher order transition-metal chalcogenides and also rare-earth metal chalcogenides at intermediate temperatures between 270 "C and 500°C was reported by Ibers and co-workers[bl. Since then a large number of new transition-metal chalcogenides were prepared"], but only a few lanthanoid compounds. These include the compounds KCeQ4 (Q = Se, Te)r81, ACuCezQ6 (A = K, Cs; Q = S, Se)[9%'01 and K2CuzCeS4[101 which all represent new structure types of multinary alkali lanthanide chalcogenides. In this paper we report on the structure and synthesis of the first quaternary alkali europium chalcogenide according to the low-temperature route. In KCuCe2Se6 the Cu centers are statistically distributed over the tetrahedral sites, whereas the Cu+ ions in the title compound occupy these sites in an ordered way leading to the formation of a superstructure of the KCuCe2Se6 structure type with all lattice parameters doubled. Results and DiscussionThe crystal structure of KCuEu2S6 is displayed in Figure 1. Figure 2 shows the numbering scheme as well as the environment of the different atoms. Double layers of Eu3+ and Cui centered polyhedra are within the ab plane. Every layer consists of interconnected EuSs and CuS4 polyhedra. The K+ ions are located in the gaps between the layers and connect these layers parallel to [OOl]. The Eu3+ ions are in a distorted bicapped trigonal prismatic environment with the short basal edges of the triangular faces being (S2)2-units and the rest of the sites

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The new tetra(alkali metal) ditantalum undecasulfides A4Ta2Sll (A = Rb, Cs) were prepared at low temperature via the molten-flux method. The crystal structures consist of well separated A + and [Ta2Sll] 4-ions. Within the anions, the Ta atoms display a sevenfold coordination by S_~-units and S 2-anions; the coordination polyhedra can be described as distorted pentagonal bipyramids. Two such bipyramids share common faces, thus forming the [Ta2Sll] 4-anion.