K2In12Se19, a Complex New Structure Type Based on Icosahedral Units of Se2−

K2In12Se19 is a new ternary chalcogenide in the system K2Se−In2Se3 (m.p. 997 K; colour: dark red) with the two components in a molar ratio of 1:6. This unusual solid shows a temperature-dependent order-disorder transition originating mainly from a dynamical disorder of In3+ at elevated temperatures. The average structure of the dynamically disordered phase at 673 K was analysed by means of high-temperature single-crystal X-ray data [a = 1393.18(5) pm, c = 1772.26(9) pm, Z = 3, R3�, hexagonal setting]. The most prominent feature of the complex solid is a partial structure of slightly distorted icosahedra formed by Se1 (central atom), Se3, and Se4. The topological arrangement of the icosahedra corresponds to a cubic close packing. A further independent partial structure enveloping the icosahedra is build up by Se2. The latter form large distorted polyhedra intermediate between icosahedra and cuboctahedra. In1, In2, and In3 occupy tetrahedral holes inside (In2) and outside the icosahedra (In1 and In3), whereas K is located in distorted octahedral holes. In2 and In3 are located in neighboured face-sharing tetrahedra showing an approximate 50% occupancy. They are alternatively occupied in the sense of a dynamical disorder (“average structure”). In1 and K are not affected directly by the disorder. Structural relations between K2In12Se19 and an earlier published unusual quaternary indium chalcogenide are discussed.