A Mössbauer effect investigation of the magnetic behaviour of (Fe,Co)S1 + x solid solutions

The Mössbauer spectra of (Fe, Co)S_{1 + x} were recorded at room temperature and 4.2 K for samples of varying composition to study the magnetic behaviour of the solid solutions. The Mössbauer spectra are split magnetically at iron concentrations above 16% Fe. For samples with less than 16%Fe, the Mössbauer spectra show no evidence of magnetic splitting down to 4.2 K. The room temperature centre shift data appear to vary continuously with composition and the hyperfine magnetic field decreases with decreasing Fe²⁺ concentration. A Mössbauer spectrum of ⁵⁷Fe:CoS at 4.2 K in an external field of 25 kOe showed no evidence of magnetic splitting beyond that caused by the applied field, indicating a net zero internal field.A high spin to low spin transition in Fe²⁺ is ruled out as being responsible for the observed magnetic behaviour on the basis of the centre shift data. The Mössbauer data are interpreted to indicate a substantial increase in electron delocalization towards the ligands as the 〈M-S〉 distance decreases with decreasing Fe²⁺concentration. This causes a reduction in the magnitude of the internal magnetic field contributions as well as a decrease of shielding of the nucleus, giving rise to the observed Mössbauer parameters.The Mössbauer spectrum of ⁵⁷Fe:CoS at room temperature is compared with the spectrum of FeS above the 6.7 GPa phase transition at room temperature. The similarities of the centre shift and the 〈M-S〉 distance in the two phases indicate that covalency may also be responsible for the observed high pressure behaviour of FeS, and not the presence of Fe³⁺ as was originally suggested.