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Valence and spin states in delafossite Ag Ni O 2 and the frustrated Jahn-Teller system A Ni <mml:mi …

2007; American Physical Society; Volume: 76; Issue: 19 Linguagem: Inglês

10.1103/physrevb.76.195122

1550-235X

J.‐S. Kang, S. S. Lee, G. Kim, Hongjoo J. Lee, H.K. Song, Y. J. Shin, Sang Wook Han, Chanyong Hwang, Min‐Cherl Jung, Hyun Joon Shin, B. H. Kim, S. K. Kwon, B. I. Min,

Advanced Condensed Matter Physics

Electronic structures of delafossite oxides $\mathrm{Ag}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$ 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 ${\mathrm{Ni}}^{2+}\text{\ensuremath{-}}{\mathrm{Ni}}^{3+}$ 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 $\mathrm{Ag}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{O}}_{2}$, 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.