Visualization of Co 3d high- and low-spin states via valence electron density

Properties of trivalent cobalt oxides are governed by the spin and orbital states of Co3+ ions, which are strongly coupled to their local coordination environments and chemical bonding. However, direct real-space access to the electronic states has remained challenging. Here, we determine the Co 3d states in the quasi-one-dimensional cobalt oxide Ca3Co2O6 by combining synchrotron X-ray diffraction with valence electron density (VED) analysis based on core differential Fourier synthesis. The reconstructed VED reveals distinct anisotropic distributions at two crystallographically inequivalent Co sites with octahedral and trigonal-prismatic coordination geometries. The octahedral site exhibits a characteristic VED consistent with a low-spin configuration, whereas the trigonal-prismatic site shows pronounced anisotropy that cannot be described solely by crystal electric field (CEF) effects. Quantitative analysis demonstrates that this anisotropy originates from the interplay of CEF effects, spin-orbit coupling, and ligand-assisted 3d-4p hybridization, reflecting partially unquenched orbital angular momentum and its role in the Ising magnetism. These results establish a general framework for understanding site-dependent electronic structure and chemical bonding in transition-metal oxides through real-space VED analysis.