Crystal structures of octacaesium and octarubidium docosaniobates

Crystals of Cs8Nb22O59 and Rb8Nb22O59are isomorphous. The structures of the two compounds have been determined by direct methods from 967 (Cs) and 791 (Rb) independent reflections, measured with a diffractometer, and refined by full-matrix least-squares methods to R 0.043 (Cs) and 0.102 (Rb). Crystals are rhombohedral (indexed on the hexagonal triple cell), space group Rm, a= 7.53(1) and c= 43.02(6)A(Cs), a= 7.53(1) and c= 43.39(6)A(Rb). The structure consists of NbO6 octahedra linked together through corners and edges, with the alkali-metal cations occupying positions which give an approximately close-packed array with the oxygen atoms.