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 ampoule s (melted at 873 K; annealed at 573 K). The compound forms brittle cry stals with silver metallic lustre, behaves as a metallic conductor, an d is very sensitive to air and moisture. The crystal structure of K3Na 26In48 (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. Th e latter are centered by Na atoms. All ln12 clusters are interconnecte d by 12 exo-bonds forming a covalent 3 D network (In-In = 2.912 - 3.14 9 angstrom). The remaining Na atoms form a Na46 3 D-network of the cla thrate-I type with (2 + 6) cages, enveloping the In12 clusters network . This novel structure can be regarded as a filled derivative of the c lathrate-I structure: Xe2Xe6(H2O)46square6 = [In12ico]2[NaIn12hap]6Na4 6K6. The potassium atoms link the filled [NaIn12hap] clusters to 1 D c olumns, and also form a (6,4) net of the NbO type with Na atoms at bri dging positions. The whole structure can also be described as a bcc pa cking of huge condensed A122 clusters (16 angstrom diameter) which hav e nearly icosahedral symmetry and consist of several atomic shells. Th e bonding as well as the structural relationships to other phases are discussed in detail.