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

K21-deltaNa2+deltaIn39 with delta = 2.82 was synthesized (melted at 97 3 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) angs trom, b = 17.192(3) angstrom, c = 25.078(7) angstrom; Z = 4; Pearson c ode oP248; delta = 2.82, obtained from the structure refinement) conta ins eight empty In-12(ico) icosahedra of two types, A (12 exo-bonds) a nd B (7 exo-bonds), and four open In15 clusters (15 exo-bonds). The la tter are centered by K atoms and belong to C units, which are defined as [K(Na2M3In15)] heteroatomic clusters (M = K + Na). The spatial dist ribution of the In-12(ico) icosahedra A, B and heteroatomic clusters C is that of the atoms in the cubic Laves phase MgCu2: MgCu2 correspond s to [K(Na2M3In15)][In-12(ico)]2. All the In(n) clusters are interconn ected by exo-bonds forming a covalent three-dimensional framework (d(I n-In) = 2.832 to 3.301 angstrom). The remaining alkali metal atoms bui ld up a three-dimensional M136 network of the clathrate-II type with ( 16 + 8) cages, which envelopes the In-12(ico) icosahedra and [K(Na2M3I n15)] clusters. This structure can be described as a cluster-replaceme nt 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 ar e discussed.