Abstract

Sulphosalts in the system CuSbS 2 -CuBiS 2 (chalcostibite-emplectite) form a complete solid solution series. Seven compositions with the general formula Cu(Sb x Bi (sub 1-x) )S 2 have been synthesized using dry methods at 310 degrees C. All members of the series are orthorhombic (space group Pnma) and show smoothly increasing a and b cell parameters with substitution of Bi for Sb; the c cell parameter increases up to 50% CuBiS 2 substitution and then becomes constant. DSC experiments on CuBiS 2 show an endothermic heat effect (2.45 kJ/mol.) at 472 degrees C due to the breakdown reaction to Cu 3 BiS 3 (wittichenite) plus Bi 2 S 3 (bismuthinite). With the addition of 10% CuSbS 2 to CuBiS 2 , the decomposition temperature increases and the endothermic peak is broadened but the energy remains essentially the same (2.53 kJ/mol.). No evidence of this decomposition was observed when the amount of the CuSbS 2 component was >30%. The local structure and co-ordination of Cu in the samples were studied by EXAFS analysis of the Cu-K edge but no significant variation occurs in the local Cu environment. The Debye-Waller factor for the first shell of S atoms surrounding Cu in end member CuSbS 2 tends to be slightly smaller than for the intermediate solid solutions, suggesting that the tetrahedral Cu environments in the intermediate composition samples is somewhat more disordered than in the end-member. The low expansion characteristics along c appear to be controlled by the linkages between the (CuS 3 +BiS 2 ) sheets perpendicular to c being relatively inflexible.

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