Revealing the site selective local electronic structure of Co${}_3$O${}_4$ using $2p3d$ resonant inelastic X-ray scattering

We investigate mixed-valence oxide Co${}_3$O${}_4$ using Co $2p3d$ resonant inelastic X-ray scattering (RIXS). By setting resonant edges at Co${}^{2+}$ and Co${}^{3+}$ ions, the $dd$ excitations on the two Co sites are probed selectively, providing detailed information on the local electronic structure of Co${}_3$O${}_4$. The $2p3d$ RIXS result reveals the ${}^4$T${}_2$ excited state of tetrahedral Co${}^{2+}$ site at 0.5 eV beyond the discriminative power of optical absorption spectroscopies. Additionally, the ${}^3$T${}_{2g}$ excited stated at 1.3 eV is uniquely identified for the octahedral Co${}^{3+}$ site. Guided by cluster multiplet simulations, the ground-state character of the Co${}^{2+}$ and Co${}^{3+}$ site is determined to be high-spin ${}^4$A${}_2$(T${}_d$) and low-spin ${}^1$A${}_{1g}$(O${}_h$), respectively. This indicates that only the Co${}^{2+}$ site is magnetically active site at low-temperatures in Co${}_3$O${}_4$. The ligand-to-metal charge transfer analysis suggests a formation of a strong covalent bonding between Co and O ions at the Co${}^{3+}$ site, while Co${}^{2+}$ is rather ionic.