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

High-accuracy single-crystal structure factor data sets, up to sinθ/λ=1.6Å^-1, have been measured from MnO at 295 K and in the antiferromagnetic state at 15 K using 316.5-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. A substantial difference is observed between the d-orbital occupancies in the two magnetic phases. The Mn valence region shows considerable asphericity already in the cubic phase. In the antiferromagnetic state, distinct anisotropies are observed both in electron density and thermal vibrations, contrary to the common belief that the distortion from cubic symmetry is negligible. The 3d charge distribution is contracted by about 4% relative to the free atom. The total number of d electrons on Mn is found to be 4.74(2), in accordance with the magnetic moment derived from neutron diffraction. Special attention is devoted to Bader’s atoms-in-molecules theory, which reveals the Mn-O interaction to be purely ionic. The implication for the interpretation of the superexchange mechanism is discussed.