Structure and magnetism in synthetic pyrrhotite Fe₇S ₈: A powder neutron-diffraction study

The relationship between structural and magnetic properties in stoichiometric pyrrhotite Fe₇S₈ has been investigated using variable-temperature neutron-diffraction data. Below its magnetic ordering temperature, T_N=598(5) K, Fe₇S₈ exhibits a monoclinic 4C structure, related to that of NiAs, in which fully occupied cation layers alternate with cation-deficient layers. The magnetic structure consists of ferromagnetically aligned layers, with antiferromagnetic coupling of adjacent layers. The ordered moment of 3.16(1)µB at 11 K is directed at an angle of 29° to the layers. The magnetic transition is accompanied by a structural transformation from the monoclinic 4C structure to a hexagonal cation-deficient NiAs structure, in which the vacancies are statistically distributed between all layers. Although the 4C structure is recovered on cooling through the magnetic transition, the resultant phase exhibits a significant degree of intralayer cation disorder.