Structures and Conductivities of the Quaternary A/Bi/Cu/S Phases KBi2CuS4 and A3Bi5Cu2S10 (A=Rb, Cs)

The compounds KBi2CuS4, Cs3Bi5Cu2S10, and Rb3Bi5Cu2S10 were prepared by stoichiometric reactions of the corresponding alkali-metal sulfides with Bi, Cu, and S at 1123 K. KBi2CuS4 crystallizes as a racemic twin in space group Cmc21 of the orthorhombic system with four formula units in a cell of dimensions a=4.0273(8), b=13.770(3), c=14.259 (3) ? at 153 K; A3Bi5Cu2S10 (A=Cs or Rb) are isostructural and crystallize in a new structure type in space group Pnnm with two formula units in cells of dimensions a=14.571 (3), b=17.819(4), c=4.0686(8) ? for Rb3Bi5Cu2S10 and a=15.041(3), b=17.994(3), c= 4.0973(8) ? for Cs3Bi5Cu2S10 at 153 K. These are channel structures with the alkali-metal cations located in channels formed by edge sharing of distorted Bi octahedra and distorted Cu tetrahedra. Electrical conductivity measurements on KBi2CuS4 and Rb3Bi5Cu2S10 indicate that both materials are semiconducting. The conductivity of KBi2CuS4 is two orders of magnitude higher than that of Rb3Bi5Cu2S10 at room temperature. KBi2CuS4 undergoes a transition from an n- to a p-type semiconductor at 210 K as determined by thermopower measurements