Abstract
There was a disagreement of chemical formulae between the formula derived from structure determination (CuBi5S8) [Ohmasa, M. and Nowacki, W. (1973) Zeits. Krist., 137, pp. 422–432] and the one from synthetic studies (CuBi3S8). In order to clarify the origin of the disagreement, the crystal structure of the compound has been reinvestigated. It was found that some of the Bi atoms are partly substituted by Cu, and that the obtained chemical formula is Cu1.57Bi4.57S8. When the number of S is fixed to five, the new formula can also be expressed as Cu0.98Bi2.86S5, and is approximately identical with CuBi3S5. The amount of substitution found in Cu2+xBi6−xS9 (x=1.21) [Ohmasa, M. (1973) Neues Jahrb. Miner. Monatsh., pp. 227–233] was also reexamined for comparison.
Both structures consist of two kinds of slabs, and the substitution between Bi and Cu was found in the slab II. Since these elements have quite different properties, the substitution found in these compounds is quite unique. The chemical formulae of the fundamental structures, which are ideal structures having no substitution between Bi and Cu, are different from the apparent formulae given in the synthetic studies, and the discrepancies are compensated by substitution, Cu for Bi. The formula CuBi5S8 obtained in the previous study corresponds to that of the fundamental structure.
