K3NA26IN48 - AN INTERMETALLIC PHASE WITH LARGE PSEUDO-ICOSAHEDRAL CLUSTERS AND A NA46 CLATHRATE-I NETWORK ENVELOPING A COVALENT (3)(INFINITY)[IN12] CLUSTER FRAMEWORK

K3Na26In48 was synthesized from the elements in sealed niobium ampoules (melted at 873 K; annealed at 573 K). The compound forms brittle crystals with silver metallic lustre, behaves as a metallic conductor, and is very sensitive to air and moisture. The crystal structure of K3Na26In48 (cubic; space group Pm3nBAR, No. 223; a = 16.046(2) angstrom; Z = 2; Pearson symbol cP154) contains two empty icosahedral In12ico and six hexagonal antiprismatic In12hap indium clusters per unit cell. The latter are centered by Na atoms. All ln12 clusters are interconnected by 12 exo-bonds forming a covalent 3 D network (In-In = 2.912 - 3.149 angstrom). The remaining Na atoms form a Na46 3 D-network of the clathrate-I type with (2 + 6) cages, enveloping the In12 clusters network. This novel structure can be regarded as a filled derivative of the clathrate-I structure: Xe2Xe6(H2O)46square6 = [In12ico]2[NaIn12hap]6Na46K6. The potassium atoms link the filled [NaIn12hap] clusters to 1 D columns, and also form a (6,4) net of the NbO type with Na atoms at bridging positions. The whole structure can also be described as a bcc packing of huge condensed A122 clusters (16 angstrom diameter) which have nearly icosahedral symmetry and consist of several atomic shells. The bonding as well as the structural relationships to other phases are discussed in detail.