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

Abstract

We have studied the electronic structure of the quasi-two-dimensional Co oxides ${\mathrm{NaCo}}_{2}{\mathrm{O}}_{4}({\mathrm{Na}}_{x}{\mathrm{CoO}}_{2},x\ensuremath{\sim}0.5\text{--}0.6),\phantom{\rule{0.2em}{0ex}}{\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$, 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 $3d\phantom{\rule{0.3em}{0ex}}{t}_{2g}$ 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 ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2},\phantom{\rule{0.2em}{0ex}}{\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9}$, and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{9}$ have different electric and magnetic properties at low temperatures.