Electronic structures of delafossite oxides $\mathrm{Ag}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_2$ and the frustrated Jahn-Teller (JT) system $A\mathrm{Ni}{\mathrm{O}}_2$ $(A=\mathrm{Li},\mathrm{Na})$ have been investigated by employing soft x-ray absorption spectroscopy and photoemission spectroscopy (PES). It is found that Ni ions are in the ${\mathrm{Ni}}^{2+}\text{−}{\mathrm{Ni}}^{3+}$ mixed-valent states and that the low-spin (LS) ${\mathrm{Ni}}^{3+}$ component increases from $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_2$ to $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_2$ and $\mathrm{Na}\mathrm{Ni}{\mathrm{O}}_2$, in agreement with the presence of the JT transition in $\mathrm{Na}\mathrm{Ni}{\mathrm{O}}_2$ and the absence of the JT transition in $\mathrm{Li}\mathrm{Ni}{\mathrm{O}}_2$ and $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_2$. In $\mathrm{Ag}{\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_2$, the ${\mathrm{Ni}}^{3+}$ component increases with $x$, while Co ions are in the LS ${\mathrm{Co}}^{3+}$ states for all $x$, which is consistent with the metallic nature for low values of $x$. A good agreement is found between the measured PES spectra and the calculated local spin density approximation (LSDA) electronic structures of $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_2$ and $\mathrm{Ag}\mathrm{Co}{\mathrm{O}}_2$, but the pseudogap feature in PES of $\mathrm{Ag}\mathrm{Ni}{\mathrm{O}}_2$ is not described by the LSDA.

• Received 16 September 2007

DOI:https://doi.org/10.1103/PhysRevB.76.195122