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 7 00 degreesC for 5 days. Single-crystal X-ray diffraction analysis reveals t hat 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. T he structure contains a three-dimensional In network built of sheets of con densed 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 cal culations and resistivity and susceptibility measurements indicate that the compound is metallic and diamagnetic. The one-electron deficiency in the v alence band per formula unit brought on by the limited cation count is anal yzed in terms of the character of the bonding, some of that from multicente r In bonding falling above E-F.