Structures and conductivities of the quaternary A/Bi/Cu/S phases KBi2CuS4 and A(3)Bi(5)Cu(2)S(10) (A=Rb, Cs)

The compounds KBi2CuS4, Cs3Bi5Cu2S10, and Rb3Bi5Cu2S10 were prepared by sto ichiometric reactions of the corresponding alkali-metal sulfides with Bi, C u, and S at 1123 K. KBi2CuS4 crystallizes as a racemic twin in space group Cmc2(1) of the orthorhombic system with four formula units in a cell of dim ensions a = 4.0273(8), b = 13,770(3), c = 14.259(3) Angstrom at 153 K; A(3) Bi(5)Cu(2)S(10) (A = Cs or Rb) are isostructural and crystallize in a new s tructure type in space group Pnnm with two formula units in cells of dimens ions a = 14.571(3), b = 17,819(4), 4.0686(8) Angstrom for Rb3Bi5Cu2S10 and a = 15.041(3), b = 17,994(3), c = 4.0973(8) Angstrom for Cs3Bi5Cu2S10 at 15 3 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 Rb3Bi5Cu2S 10 indicate that both materials are semiconducting. The conductivity of KBi 2CuS4 is two orders of magnitude higher than that of Rb3Bi5Cu2S10 at room t emperature. KBi2CuS4 undergoes a transition from an n- to a p-type semicond uctor at 210 K as determined by thermopower measurements, (C) 2000 Academic Press.