Mössbauer Effect in the System Co1−xFexS2

X‐ray diffraction analysis and magnetic‐susceptibility measurements of samples in the system, Co_1−xFe_xS₂, where $\text{x\hspace{0.17em}=\hspace{0.17em}0.01}$ , 0.25, 0.50, and 0.75, indicate formation of a solid‐solution series having the pyrite structure. These compounds are ferromagnetic with Curie temperatures in the range of 0°–125°K. In spite in the appearance of long‐range magnetic order, Mössbauer effect measurements of ⁵⁷Fe at 300°, 100°, and 5°K do not exhibit magnetic hyperfine splitting. This is interpreted as evidence that the divalent iron ions remain in the low spin state throughout the system and that the magnetic ordering process involves electrons localized on the cobalt ions. Trends in isomer shift and quadrupole splitting are discussed relative to changes in the lattice constant.