The crystal chemistry and magnetic properties of iron in the monosulfide solid solution of the Fe-Ni-S system

Compositions of varying iron:nickel and metal:sulfur ratios occurring within the monosulfide solid solution (Mss) of the Fe-Ni-S system have been synthesized at 600°C. The samples were characterized by X-ray diffraction and electron microprobe analysis, and the crystal chemistry and magnetic properties of iron in the samples studied by Mössbauer spectroscopy. Compositions close to the FeS corner of the Mss have a six-peak Mössbauer spectrum indicating antiferromagnetically ordered iron in one distinct site. Compositions across the center of the Mss ((Fe, Ni)_0.966S) exhibit similar six-peak spectra until ≲ 25 wt percent iron when a magnetic order ⇄ disorder transition occurs. Samples with < 20 wt percent iron exhibit simple doublet spectra with very small isomer shifts (~ 0.50 mm/sec) and quadrupole splittings (~ 0.40 mm/sec) for Fe²⁺ in octahedral sites. These low values probably are due to electron delocalization and metallic behavior at the Ni-rich end of the Mss. Samples having still lower metal content (~M_0.91-0.95S) exhibit the order ⇄ disorder transition at ~ 35 wt percent iron, and have more complex magnetic spectra due to vacancy ordering. The magnetic order ⇄ disorder transition occurs in regions of the Mss where it breaks on cooling. Discontinuities occur in d(102) and c vs composition plots in these regions and have been used to support a previous proposal of a “modulated structure” in these regions.