Indications of the magnetic state in the charge distributions in MnO, CoO, and NiO. I: Para-and antiferromagnetism of MnO

X-ray diffraction intensities from MnO and CoO were measured above and below their Néel temperatures and from NiO, below the Néel temperature. To detect possible characteristics of the paramagnetic and antiferromagnetic states of the crystals, the data were subjected to direct multipole analysis of the atomic-charge densities. For MnO, both spherical and nonspherical accumulation-of-charge densities indicate the exchange of the roles played by manganese and oxygen in the magnetic phase transition. Both spherical and nonspherical features characteristic of the ionic nature are inherent in both states. The electron counts of the density peaks correspond to Mn²⁺ and O^1?, with the tenth electron of O2? being distributed in a wider region. In the paramagnetic state, there is an electronic Mn-Mn bond which seems to be formed due to coupling with the tenth electron of O^2? and builds up a three-dimensional net of the charge density with the “cages” surrounding oxygen atoms. In the antiferromagnetic state, some Mn-Mn bonds disappear, while the preserved nonspherical ionic features enhance the role of oxygen atoms in electronic coupling.