Alkaline metal arsenides A3As11 (A=Rb, Cs):: Preparation and crystal structures

Rb3As11 and Cs3As11 were synthesized from the elements and the crystal structures of the ordered room temperature form were characterized via single crystal x-ray studies. In the Zintl phases the As atoms form chiral ufosan-anions As-11(3-) with As-As distances ranging from 238 to 248 pm. Like K3As11 Rb3As11 crystallizes with the Na3P11 structure type (orthorhombic, space group Pbcn, a = 1108.2(2), b = 1533.5(3), c = 1060.1(2) pm, Z = 4), whereas the Cs compound (monoclinic, space group C2/c, a = 1324.5(7), b = 1524.5(9), c = 1937.2(11) pm, beta = 95.29(1)degrees, Z = 8) forms a new structure type. The crystallographic relationship between the two structure types and the anion packings in the plastic crystalline high temperature forms are discussed.