Mössbauer study of bornite and chemical bonding in Fe-bearing sulphides

The Mössbauer spectra of a nearly stoichiometric natural bornite, Cu5FeS4, specimen were reinvestigated between 295 and 4.2 K. There is no difference between the Neél temperature TN as determined by the Mössbauer effect or by the susceptibility measurements (TN = 67.5 K). No additional paramagnetic doublet can be observed in the low-temperature MS spectra. The valence state of Fe is Fe(3−x)+ caused by a partial electron transfer from the Cu+ ions to the Fe3+ ions which increases the shielding of the s-electrons by the d-electron density and by this increases the isomer shift to a value intermediate between tetrahedral high-spin Fe2+ and tetrahedral high-spin Fe3+.