Synthesis, crystal and electronic structures of the new quaternary phases A5Cd2Sb5F (A = Sr, Ba, Eu), and Ba5Cd2Sb5O(x) (0.5<x<0.7).

A new family of quaternary fluoro-antimonides A(5)Cd(2)Sb(5)F (A = Sr, Ba, Eu) and oxyantimonides Ba(5)Cd(2)Sb(5)O(x) (0.5<x<0.7) have been synthesized and structurally characterized. The title compounds are isotypic, and crystallize with a novel structure type in the orthorhombic space group Cmcm (Z = 4, Pearson symbol oC52). The crystal structures feature one-dimensional double-strands of corner-shared CdSb(4) tetrahedra, [Cd(2)Sb(5)](9-). The arrangement of the chains is such that two of the unshared Sb vertices are close together to form a Sb-Sb bond, effectively making pentagonal quasi-channels. One of the five A(2+) cations reside inside the tubes with the remaining four surrounding them. The electron balance is provided by O(2-) or F(-) anions, which fill suitable tetrahedral holes, left between the outer A(2+) cations. The chemical bonding in the new family has been investigated using TB-LMTO-ASA band structure calculations, which suggest that the presence of the electron acceptor, oxygen or fluorine, is essential for optimization of the bonding interactions.