Electron density distribution in hexagonal cobalt: Aγ-ray diffraction study

Abstract

The electron density distribution of ferromagnetic hexagonal cobalt is studied at room temperature using high-quality single-crystal diffraction data measured up to $\text{sin}\text{ }\ensuremath{\theta}/\ensuremath{\lambda}=1.9\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ 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 ${a}_{\text{g}}$ along the $c$ axis and a deficiency in the two doublet ${e}_{\text{g}}$ 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 $3{d}^{7}$ configuration implies incomplete filling of the majority-spin band. The directed metallic bonds are characterized in terms of the electron density topology.