K21-DELTA-NA2+DELTA-IN39 (DELTA=2.8) - A CLUSTER-REPLACEMENT CLATHRATE-II STRUCTURE WITH AN ALKALI-METAL M(136)-NETWORK

K21-deltaNa2+deltaIn39 with delta = 2.82 was synthesized (melted at 973 K, annealed at 623 K) from the elements in sealed niobium ampoules. The compound forms prismatic crystals with silver metallic lustre and is unstable in air and moisture. The crystal structure of K21-deltaNa2+deltaIn39 (orthorhombic; space group Pnma, No. 62; a = 17.844(5) angstrom, b = 17.192(3) angstrom, c = 25.078(7) angstrom; Z = 4; Pearson code oP248; delta = 2.82, obtained from the structure refinement) contains eight empty In-12(ico) icosahedra of two types, A (12 exo-bonds) and B (7 exo-bonds), and four open In15 clusters (15 exo-bonds). The latter are centered by K atoms and belong to C units, which are defined as [K(Na2M3In15)] heteroatomic clusters (M = K + Na). The spatial distribution of the In-12(ico) icosahedra A, B and heteroatomic clusters C is that of the atoms in the cubic Laves phase MgCu2: MgCu2 corresponds to [K(Na2M3In15)][In-12(ico)]2. All the In(n) clusters are interconnected by exo-bonds forming a covalent three-dimensional framework (d(In-In) = 2.832 to 3.301 angstrom). The remaining alkali metal atoms build up a three-dimensional M136 network of the clathrate-II type with (16 + 8) cages, which envelopes the In-12(ico) icosahedra and [K(Na2M3In15)] clusters. This structure can be described as a cluster-replacement derivative of the clathrate-II structure: (H2S)16(CCl4)8 . (H2O)136 corresponds to [In-12(ico)]16[K(M5In15)]8M136, and is one member of a novel hierarchical structure family, based upon cluster-replacement. The bonding as well as the structural relationships to other phases are discussed.