New members of the homologous series A_m[M₆Se₈]_m [M_5+nSe_9+n]: The quaternary phases A_1-xM′_3-xBi_11+xSe₂₀ and A_1+xM′_3-2xBi_7+xSe₁₄ (A = K, Rb, Cs; M′ = Sn, Pb)

The compounds A_1+xM′_3-2xBi_7+xSe₁₄ and A_1-xM′_3-xBi_11+xSe₂₀ (A = K, Rb, Cs; M′ = Sn, Pb) were discovered from reactions involving A₂Se, Bi₂Se₃, M′, and Se at 760 °C. The single-crystal structures reveal that A_1+xM′_3-2xBi_7+xSe₁₄ are isostructural to K_2.5Bi_8.5Se₁₄ whereas A_1-xM′_3-xBi_11+xSe₂₀ adopt a new structure type. Both compound types belong to the homologous series A_m[M₆Se₈]_m [M_5+nSe_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 NACI lattice, perpendicular to the [111] (NaCl¹¹¹-type) and [100] (NaCl¹⁰⁰-type) directions, respectively, with dimensions and shapes defined by m and n. The structures of A_1+xM′_3-2xBi_7+xSe₁₄ (m = 2, n = 3) and A_1-xM′_3-xBi_11+xSe₂₀ (m = 1, n = 3) exhibit the same type of stepshaped NACl¹¹¹-type layer but differ in the size of the NaCl¹⁰⁰-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+xM′_3-2xBi_7+xSe₁₄ and A_1-xM′_3-xBi_11+xSe₂₀ (A = K, Rb, Cs; M′ = Sn, Pb) are reported.