Electron density distribution in ferromagnetic nickel: A γ-ray diffraction study

High-accuracy single-crystal structure factors, complete up to sin theta/lambda=1.9 A(-1), have been measured from ferromagnetic nickel at 295 K using 316.5-keV gamma radiation. The experimental uncertainty of the structure factors is of the order of 10 millielectrons per atom for all data. A detailed description of the electron density distribution is presented in terms of a multipolar atomic deformation model. Achievement of a reliable Debye-Waller factor is of vital importance in this context. The charge asphericity is due to an excess e(g) orbital occupancy of 43.4(2)%. The 3d shell in the metal is contracted by 2.07(5)% relative to the free atom. The results are discussed and compared with earlier experimental and theoretical works. In contrast to bcc Cr and Fe, solid-state effects are less pronounced in fcc Ni. Clear disentanglement between the 3d and 4s valence electrons could be accomplished for the first time. The general expectation that the number of 3d electrons in the metal should be increased as compared to the atom was confirmed in the case of iron by combining spin and charge-density data. In the case of nickel, it is rejected as revealed by the gamma-ray data alone. Only with the d(8) configuration, consistency is achieved between observed and refined mosaic widths of the sample crystal. A 3d(8) configuration implies that the majority-spin d band cannot be full. Strong support is lent to a localized atomic character of the valence electrons.