Electron density distribution in hexagonal cobalt: A γ-ray diffraction study

The electron density distribution of ferromagnetic hexagonal cobalt is studied at room temperature using high-quality single-crystal diffraction data measured up to sin theta/lambda = 1.9 angstrom(-1) with 316.5 keV gamma radiation. A highly anisotropic mosaic-block orientation was found. The ensuing anisotropy in secondary extinction could be fairly well described by the Thornley-Nelmes formalism, with adjusted mosaicities close to the directly observed ones. The structure factors have been analyzed by a multipole expansion model within the Hartree-Fock framework. Thermal mean-square atomic amplitudes are distinctly different along the a and c axes. The total 3d charge exhibits appreciable anisotropy with an excess of density in the singlet a(g) along the c axis and a deficiency in the two doublet e(g) orbitals. The 3d shell in the metal shows a slight contraction of 1.2% relative to the free atom. Agreement with low-order form factors from critical-voltage electron diffraction reaches 0.1%. No difference of d-electron count is found between metal and atom. A 3d(7) configuration implies incomplete filling of the majority-spin band. The directed metallic bonds are characterized in terms of the electron density topology.