Copper sulfide

Copper sulfide

Copper sulfides describe a family of chemical compounds and minerals with the formula CuxSy. Both minerals and synthetic materials comprise these compounds. Some copper sulfides are economically important ores.

Prominent copper sulfide minerals include Cu2S (chalcocite) and CuS (covellite). In the mining industry, the minerals bornite or chalcopyrite, which consist of mixed copper-iron sulfides, are often referred to as "copper sulfides". In chemistry, a "binary copper sulfide" is any binary chemical compound of the elements copper and sulfur. Whatever their source, copper sulfides vary widely in composition with 0.5 ≤ Cu/S ≤ 2, including numerous of non-stoichiometric compounds.

Known copper sulfides

The binary compounds of copper and sulfur are listed below. There are probably more yet to be discovered, for example investigations of "blaubleibender covellite" (blue remaining covellite) formed by natural leaching of covellite,(CuS) indicate that there are other metastable Cu-S phases still to be fully characterised.

Classes of copper sulfides

Copper sulfides can be classified into three groups:

Monosulfides, 1.6 ≤ Cu/S ≤ 2: their crystal structures consist of isolated sulfide anions that are closely related to either hcp or fcc lattices, without any direct S-S bonds. The copper ions are distributed in a complicated manner over interstitial sites with both trigonal as well as distorted tetrahedral coordination and are rather mobile. Therefore, this group of copper sulfides shows ionic conductivity at slightly elevated temperatures. In addition, the majority of its members are semiconductors.

Mixed monosulfide and disulfide compounds of copper contain both monosulfide (S2−) as well as disulfide (S2)n− anions. Their crystal structures usually consist of alternating hexagonal layers of monosulfide and disulfide anions with Cu cations in trigonal and tetrahedral interstices. CuS, for example, can be written as Cu3(S2)S. Several nonstoichiometric compounds with Cu:S ratios between 1.0 and 1.4 also contain both monosulfide as well as disulfide ions. Depending on their composition, these copper sulfides are either semiconductors or metallic conductors.

At very high pressures, a copper disulfide, CuS2, can be synthesized. Its crystal structure is analogous to that of pyrite, with all sulfur atoms occurring as S-S units. Copper disulfide is a metallic conductor due to the incomplete occupancy of the sulfur p band.

Oxidation states of copper and sulfur

The bonding in copper sulfides cannot be correctly described in terms of a simple oxidation state formalism because the Cu-S bonds are rather of covalent than ionic character and have a high degree of delocalization, resulting in complicated electronic band structures. Although many textbooks (e.g. ) give a mixed valence formula (Cu+)2(Cu2+)(S2–)(S2)2– for CuS, X-ray photoelectron spectroscopic data give strong evidence that, in terms of the simple oxidation state formalism, all the known copper sulfides should be considered as purely-monovalent copper compounds, and more appropriate formulas would be (Cu+)3(S2–)(S2) for CuS, and (Cu+)(S2) for CuS2, respectively. Further evidence that the assignment of the so-called "valence hole" to the S2 units in these two formulas is the length of the S-S bonds that are significantly shorter in CuS (0.207 nm) and CuS2 (0.203 nm) than in the "classical" disulfide Fe2+(S2)2− (0.218 nm). This bond length difference has been ascribed to the higher bond order in (S-S) compared to (S-S)2− due to electrons being removed from a π* antibonding orbital. NMR studies on CuS show that there are two distinct species of copper atom, one with more metallic nature than the other. and this apparent discrepancy with the X-ray photo-electron spectrum data simply highlights the problem of assigning oxidation states in a mixed valence compound.

References

  • J.C.W. Folmer Holes in the valence band of copper chalcogenides Thesis 1981 Groningen State University (Neth).

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