Structure and thermoelectric properties of the new quaternary bismuth selenides A(1-x)M(4-x)Bi(11+x)Se(21) (A = K and Rb and Cs; M = Sn and Pb) - Members of the grand homologous series K-m(M6Se8)(m)(M5+nSe9+n)

Several members of the new family A(1-x)M(4-x)Bi(11+x)Se(21) (A=K, Rb, Cs; M = Sn, Pb) were prepared by direct combination of A,Se, Bi,Se,, Sn (or Pb) , and Se at 800 degreesC. The single-crystal structures of K0.54Sn3.54Bi11. 46Se21, K-1.46 Pb3.08Bi11.46Se21, Rb0.69Pb3.69Bi11.31Se21, and Cs0.65Pb3.65 Bi11.35Se21 were determined. The compounds A(1-x)M(4-x)Bi(11+x) Se,, crysta llize in a new structure type with the monoclinic space group C2/m, in whic h building units of the Bi2Te3 and NaCl structure type join to give rise to a novel kind of three-dimensional anionic framework with alkali-ion-filled tunnels. The building units are assembled from distorted, edge-sharing (Bi ,Sn)Se-6 octa- hedra. Bi and Sn/Pb atoms are disordered over the metal site s of the chalcogenide network, while the alkali site is not fully occupied. A grand homologous series K-m(M6Se8)(m)(M5+nSe9+n) has been identified of which the compounds A(1-x) M4-xBi11+xSe21 are members. We discuss here the crystal structure, charge-transport properties, and very low thermal conduc tivity of A(1-x)M(4-x)Bi(11+x)Se(21).