γ-ray and neutron diffraction studies of CoF₂: magnetostriction, electron density and magnetic moments

Acta Cryst. (2004). A60, 51–57 [doi:10.1107/S0108767303022803]

Contour map of the model deformation electron density with Co being located in the center. Preferred occupation of 3d orbitals gives rise to excess densities as large as 4 eÅ^-3.

Accurate structure factors up to sinθ/λ = 1.6 Å^-1 have been measured from CoF₂ at room temperature and in the antiferromagnetic state at 10 K using 316.5 keV gamma radiation. Extended time-of-flight neutron diffraction data allowed an accurate determination of the fluorine positional parameter. A complete charge-density analysis has been carried out for the ordered phase. On magnetic ordering, a small shift of the fluorine ions is found which confirms a prediction from theory of optical birefringence. The effect of magnetostriction on the distortion of the ligand coordination octahedra is compared for the late members of the 3d transition-metal difluorides, and is shown to follow a remarkably simple scheme. From neutron powder diffraction, an ordered magnetic moment of 2.60(4) μ_B per cobalt ion is found. Despite the strong deviation from the spin value of 3 μ_B for the free atom there is still an appreciable orbital contribution to the local magnetic moment.