RbSb2 - A Zintl phase related to KSb2

The electron-precise Zintl compound RbSb2, which was known to melt incongruently at 418 degrees C, has been prepared in pure phase from elemental rubidium and antimony in sealed tantalum crucibles. In accordance with the ribbon-shaped antimonide anions, the compound crystallizes with extremely thin intergrown, mechanically and chemically very sensitive needles of dark-metallic lustre. The crystal structure could be determined and refined using single crystal x-ray data (monoclinic, space group C2lm, a = 1403(2), b = 414.0(4), c = 855.7(14) pm, beta = 104.45(12)degrees, Z = 4, Rl = 0.0901) despite the poor quality of the crystals. It shows fused six-membered rings of two- and three-bonded Sb atoms forming ribbons running along the monoclinic b axis. which can be interpreted as sections of the elemental structure of antimony (d(Sb-Sb) = 281.9(5) and 286.0(9) pin respectively). The structure of RbSb2 is thus closely related to that of KSb2, which exhibits identical antimony anions. Compared to the potassium compound, the (1)(x)[Sb-2](-) ribbons are reoriented against each so that the coordination number of the A counter ions is increased from 6 + 2 (for A = K) to 8 + 2 (for A = Rb). The results of a FP-LAPW band structure calculation of RbSb2 are used to explain the chemical bonding in this classical Zintl phase with a calculated indirect band gap of 0.38 eV.