Electron density distribution in paramagnetic and antiferromagnetic NiO: A γ -ray diffraction study

High-accuracy single-crystal structure factor data sets, complete up to sin0.2em0exθ∕λ=1.60.3em0ex^-1, have been measured from paramagnetic NiO at 5500.3em0exK and in the antiferromagnetic state at 100.3em0exK using 316.50.3em0exkeV gamma radiation. In the rhombohedral low-temperature phase, monodomain formation was enforced by application of moderate stress. A detailed description of the electron density distribution is derived in terms of a multipolar atomic deformation model. Pronounced asphericity is found in the nickel valence region which, unexpectedly, is not significantly influenced by the magnetic order. NiO thus exhibits a radically different behavior than that found for MnO or CoO. Similarly, very different charge responses of the eg and t2g subshells in the paramagnetic states are observed, supporting a prediction from theory. The $3d$ charge distribution is contracted by about 3% relative to the free atom; the total number of d electrons on nickel amounts to 7.65(1). From the $3d$ population analysis, unquenched orbital angular momentum is derived, in quantitative accordance with magnetic x-ray measurements. Electronic properties at the bond critical points reveal the Ni-O interactions as purely ionic. Methodological issues such as the importance of high momentum data and the influence of the wave-function quality are also discussed.