Electronic structure of ${\mathrm{Ca}}_3\mathrm{Co}X{\mathrm{O}}_6$ ($X=\mathrm{Co}$, Rh, Ir) studied by x-ray photoemission spectroscopy

We have obtained Co $2p$, Rh $3d$, Ir $4f$, and valence-band x-ray photoemission spectra of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$, ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$, and ${\mathrm{Sr}}_3\mathrm{Zn}\mathrm{Ir}{\mathrm{O}}_6$ in order to determine the valence states of Co. The spectral features are consistent with the picture that Co is in the trivalent state at both trigonal prismatic and octahedral sites with high- and low-spin states, respectively, in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, whereas Co is in the high-spin ${\mathrm{Co}}^{2+}$ state for ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$ and ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$. This conclusion resolves a recent controversy regarding the valence and spin states of Co in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ and ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$. This is the first x-ray photoemission spectroscopic study of this family of oxides.

Figure 1

Co $2p$ photoemission spectra of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$, and ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$. The calculated spectra are shown by solid curves. The Co $2p$ spectra of ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$ and ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$ are overlaid with that of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ to show the difference in the satellite region.

Figure 2

Ir $4f$ photoemission spectra of ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$ and ${\mathrm{Sr}}_3\mathrm{Zn}\mathrm{Ir}{\mathrm{O}}_6$.

Figure 3

Rh $3d$ photoemission spectra of ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$.

Figure 4

Valence-band photoemission spectra of ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$, ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Rh}{\mathrm{O}}_6$, ${\mathrm{Ca}}_3\mathrm{Co}\mathrm{Ir}{\mathrm{O}}_6$, and ${\mathrm{Sr}}_3\mathrm{Zn}\mathrm{Ir}{\mathrm{O}}_6$.