New Members of the Homologous Series Am[M6Se8]m[M5+nSe9+n]: The Quaternary Phases A1-xM‘3-xBi11+xSe20 and A1+xM‘3-2xBi7+xSe14 (A = K, Rb, Cs; M‘ = Sn, Pb)

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Abstract
The compounds A(1+x)M'(3-2x)Bi(7+x)Se(14) and A(1-x)M'(3-x)Bi(11+x)Se(20) (A = K, Rb, Cs; M' = Sn, Pb) were discovered from reactions involving A(2)Se, Bi(2)Se(3), M', and Se at 760 degrees C. The single-crystal structures reveal that A(1+x)M'(3-2x)Bi(7+x)Se(14) are isostructural to K(2.5)Bi(8.5)Se(14) whereas A(1-x)M'(3-x)Bi(11+x)Se(20) adopt a new structure type. Both compound types belong to the homologous series A(m)[M(6)Se(8)](m)[M(5+n)Se(9+n)] (M = di- and trivalent metal), whose characteristics are three-dimensional anionic frameworks with tunnels filled with alkali ions. The building units are derived from different sections of the NaCl lattice, perpendicular to the [111] (NaCl(111)-type) and [100] (NaCl(100)-type) directions, respectively, with dimensions and shapes defined by m and n. The structures of A(1+x)M'(3-2x)Bi(7+x)Se(14) (m = 2, n = 3) and A(1-x)M'(3-x)Bi(11+x)Se(20) (m = 1, n = 3) exhibit the same type of step-shaped NaCl(111)-type layer but differ in the size of the NaCl(100)-type unit. In both structures, the Bi and Sn (Pb) atoms are extensively disordered over the metal sites of the chalcogenide network. The physicochemical and charge transport properties of A(1+x)M'(3-2x)Bi(7+x)Se(14) and A(1-x)M'(3-x)Bi(11+x)Se(20) (A = K, Rb, Cs; M' = Sn, Pb) are reported.