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 NaCo2 O4 (Nax CoO2, x∼0.5-0.6), Ca3 Co4 O9, and Bi2 Sr2 Co2 O9 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 3d t2g 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 Nax CoO2 is different from that of Ca3 Co4 O9 and Bi2 Sr2 Co2 O9. We argue that the difference of the orbital anisotropy and hole concentration are essential to explain why Nax CoO2, Ca3 Co4 O9, and Bi2 Sr2 Co2 O9 have different electric and magnetic properties at low temperatures.