Structuresand 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) Å 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) Å for Rb₃Bi₅Cu₂S₁₀ and a = 15.041(3), b = 17.994(3), c = 4.0973(8) Å, 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. (C) 2000 Academic Press.