K2In12Se19 is a new ternary chalcogenide in the system K2Se-In2Se3 (m.p. 99
7 K; colour: dark red) with the two components in a molar ratio of 1:6. Thi
s unusual solid shows a temperature-dependent order-disorder transition ori
ginating mainly from a dynamical disorder of In3+ at elevated temperatures.
The average structure of the dynamically disordered phase at 673 K was ana
lysed by means of high-temperature single-crystal X-ray data [a = 1393.18(5
) pm, c = 1772.26(9) pm, Z = 3, R (3) over bar, hexagonal setting]. The mos
t prominent feature of the complex solid is a partial structure of slightly
distorted icosahedra formed by Set (central atom), Se3, and Se4. The topol
ogical arrangement of the icosahedra corresponds to a cubic close packing.
A further independent partial structure enveloping the icosahedra is build
up by Set. The latter form large distorted polyhedra intermediate between i
cosahedra and cuboctahedra. In1, In2, and In3 occupy tetrahedral holes insi
de (In2) and outside the icosahedra (In1 and In3), whereas K is located in
distorted octahedral holes. In2 and In3 are located in neighboured face-sha
ring tetrahedra showing an approximate 50% occupancy. They are alternativel
y occupied in the sense of a dynamical disorder ("average structure"). In1
and K are not affected directly by the disorder. Structural relations betwe
en K2In12Se19 and an earlier published unusual quaternary indium chalcogeni
de are discussed.