Crystal structures of octacaesium and octarubidium docosaniobates

Abstract

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)Å(Cs), a= 7.53(1) and c= 43.39(6)Å(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.