K2SrIn7:: An electron-deficient indium network structure that reflects limitations of cation accommodation.: Synthesis, structure, and bonding

A new phase in the K-Sr-In system was discovered following direct fusion of the neat elements in a niobium tube at 900 degreesC and equilibration at 700 degreesC for 5 days. Single-crystal X-ray diffraction analysis reveals that K2SrIn7 crystallizes in an orthorhombic system, space group Cmcm, Z = 4, a = 5.0455(5) Angstrom, b = 11.960(2) Angstrom, c = 19.762(4) Angstrom. The structure contains a three-dimensional In network built of sheets of condensed pentagonal prismatic columns interbonded along (c) over right arrow. Two rather different types of channels are separately occupied by K and Sr atoms, the latter centered in 15-atom indium polyhedra. Band structure calculations and resistivity and susceptibility measurements indicate that the compound is metallic and diamagnetic. The one-electron deficiency in the valence band per formula unit brought on by the limited cation count is analyzed in terms of the character of the bonding, some of that from multicenter In bonding falling above E-F.