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

High-accuracy single-crystal structure factor data sets, complete up to $\mathrm{sin}\phantom{\rule{0.2em}{0ex}}\ensuremath{\theta}∕\ensuremath{\lambda}=1.6\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$, have been measured from paramagnetic NiO at $550\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in the antiferromagnetic state at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ using $316.5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ 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 ${e}_{g}$ and ${t}_{2g}$ 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.

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