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 similar to 0.5-0.6), Ca3Co4O9, 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 3d t(2g) shell is estimated to be about 0.4, 0.6, and 0.33, respectively. The 0 Is 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 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 NaxCoO(2), Ca3Co4O9, and Bi2Sr2Co2O9 have different electric and magnetic properties at low temperatures.