Synchrotron-Radiation Study of Valence States and Electronic Structures of $\hbox {AgNi}_{1-{\rm x}}\hbox {Co}_{\rm x}\hbox {O}_{2}$ Delafossite Oxides

The electronic structures of AgNi_1-xCo_xO₂ (0 les x les 1) delafossite oxides have been investigated by employing photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) in the soft X-ray regime of synchrotron radiation. The Ag 3d core-level PES spectra shift toward the higher binding energy with increasing x, in agreement with the metallic nature for low values of x and the insulating nature for high values of x in AgNi_1-xCo_xO₂ . Further, the Ag 3d core-level PES spectra become more asymmetric with decreasing x, which reflects the larger number of electron-hole pairs in the Ni-rich regime. Valence-band PES spectra reveal that the band width of the occupied Ni 3d states is broader than that of the Co 3d states, implying the larger Ni 3d-O 2p hybridization than the Co 3d-O 2p hybridization. The Co and Ni 2p XAS spectra show that Co ions are in the low-spin Co³⁺ states for all x, while Ni ions are in the Ni²⁺-Ni³⁺ mixed-valent states. The Ni³⁺ component increases with x.