X-ray absorption study of layered Co oxides with a Co-O triangular lattice

We have studied the electronic structure of the quasi-two-dimensional Co oxides ${\mathrm{NaCo}}_2{\mathrm{O}}_4({\mathrm{Na}}_x{\mathrm{CoO}}_2,x\sim 0.5\text{–}0.6),{\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$, and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_9$ using O $1s$ and Co $2p$ x-ray absorption (XAS) spectroscopy. We found that these Co-O triangular lattice systems have in common that their ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are all low-spin, supporting the Koshibae-Tsutsui-Maekawa theory to explain the enhanced thermopower at elevated temperatures. The concentration of holes in the Co $3d{t}_{2g}$ shell is estimated to be about 0.4, 0.6, and 0.33, respectively. The O $1s$ XAS spectra strongly depend on the direction of polarization vector of the incoming x-ray. The polarization dependence indicates that the $t_{2g}$ orbital anisotropy of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ is different from that of ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_9$. We argue that the difference of the orbital anisotropy and hole concentration are essential to explain why ${\mathrm{Na}}_x{\mathrm{CoO}}_2,{\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$, and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_9$ have different electric and magnetic properties at low temperatures.

Figure 1

A schematic drawing of the Co-O triangular lattice in ${\mathrm{Na}}_x{\mathrm{CoO}}_2$, ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$, and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_9$. The Co-O triangular lattice consists of edge-sharing ${\mathrm{CoO}}_6$ octahedra. The low-spin configurations for ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ ions are also indicated.

Figure 2

O $1s$ XAS and Co $2p$ XAS spectra of ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ as a function of incidence angle.

Figure 3

O $1s$ XAS and Co $2p$ XAS spectra of ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ as a function of incidence angle.

Figure 4

Co $2p$ XAS spectra of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Co}}_2{\mathrm{O}}_9$, ${\mathrm{Na}}_x{\mathrm{CoO}}_2$, and ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ as a function of incidence angle.