Structure and magnetism in synthetic pyrrhotite<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Fe</mml:mtext></mml:mrow><mml:mn>7</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>8</mml:mn></mml:msub></mml:mrow></mml:math>: A powder neutron-diffraction study

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, ${T}_{\mathrm{N}}=598(5)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, ${\text{Fe}}_{7}{\mathrm{S}}_{8}$ 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 $3.16(1){\ensuremath{\mu}}_{\mathrm{B}}$ at $11\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is directed at an angle of $29\ifmmode^\circ\else\textdegree\fi{}$ 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.

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