Synchrotron-radiation study of valence states and electronic structures of Ag[Ni.sub.1-x][Co.sub.x][O.sub.2] delafossite oxides

The electronic structures of Ag[Ni.sub.1-x][Co.sub.x][O.sub.2] (0 [less than or equal to] x [less than or equal to] 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 Ag[Ni.sub.1-x][Co.sub.x][O.sub.2]. 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 [Co.sup.3+] states for all x, while Ni ions are in the [Ni.sup.2+] - [Ni.sup.3+] mixed-valent states. The [Ni.sup.3+] component increases with x. Index Terms--Delafossite, electronic structure, PES, synchrotron radiation, XAS.