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 NaCo₂O₄(NaₓCoO₂,x~0.5--0.6),0.2em0exCa₃Co₄O₉, and Bi₂Sr₂Co₂O₉ 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 Co³⁺ and Co⁴⁺ 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 3d0.3em0ext2g 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 t2g orbital anisotropy of NaₓCoO₂ is different from that of Ca₃Co₄O₉ and Bi₂Sr₂Co₂O₉. We argue that the difference of the orbital anisotropy and hole concentration are essential to explain why NaₓCoO₂,0.2em0exCa₃Co₄O₉, and Bi₂Sr₂Co₂O₉ have different electric and magnetic properties at low temperatures.