Compounds with pentelide dumbbells M2: AI11M6 and AII11M10 (A = Rb, Cs, Ba; M = Sb, Bi)

The new ternary alkali pentelides A(11)M(6) (A = Rb, Cs; M = Sb/Bi) all contain non electron-precise isolated dumbbells statistically composed of Sb/Bi. Their phase ranges as determined from single crystal data were found to be on the Sb-rich side of the overall composition A(11)Sb(6-x)Bi(x) in the case of the Rb compound (i.e. Rb11Sb5.4Bi0.6: orthorhombic Immm, a = 766.7(3), b = 1052.2(3), c = 1732.7(6) pm, Z = 4, R1 = 0.0554), and on the Bi-rich side of the overall composition Al11Sb6-xBix in the case of A = Cs (i.e. Cs11Sb1.6Bi4.4). In the series of known alkaline earth compounds A(11)M(10), containing pentelide dumbbells among isolated Zintl anions, a redetermination of the structure of Ba11Sb10 (orthorhombic, Immm, a = 1265.3(2), b = 1316.1(3), c =1947.2(5) pm, Z = 4, R1 = 0.0724) shows, that it represents a distorted variant of the tetragonal Ho11Ge10 structure type. This distortion is not only of crystallographic importance, as it results in a major change in the nature of the anions and their bonding, which is supported by DFT band-structure calculations. Likewise, the Zintl phase BaSb2 (monoclinic, P2(1)/m, a = 1167.9(9), b = 438.1(5), c = 1257.1(9) pm, beta = 100.53(2)degrees, Z = 6, R1 = 0.0648) crystallizes with a superstructure of the CaSb2-type previously reported.