The relationship between structural and magnetic properties in stoichiometric pyrrhotite ${\text{Fe}}_7{\mathrm{S}}_8$ has been investigated using variable-temperature neutron-diffraction data. Below its magnetic ordering temperature, $T_{\mathrm{N}}=598\left(5\right)\mathrm{K}$, ${\text{Fe}}_7{\mathrm{S}}_8$ 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 $3.16\left(1\right){\mu }_{\mathrm{B}}$ at $11\mathrm{K}$ 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.

• Received 20 October 2003

DOI:https://doi.org/10.1103/PhysRevB.70.014415