title: New members of the homologous series A(m)[M(6)Se(8)](m)[M(5+n)Se(9+n)]: The quaternary phases A(1-x)M'(3-x)Bi(11+x)Se(20) and A(1+x)M'(3-2x)Bi(7+x)Se(14) (A = K, Rb, Cs; M' = Sn, Pb).
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.
authors.affiliation: Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824, USA.
authors.forename: A
authors.initials: A
authors.lastname: Mrotzek
authors.forename: L
authors.initials: L
authors.lastname: Iordanidis
authors.forename: M G
authors.initials: MG
authors.lastname: Kanatzidis