X-ray absorption spectroscopy of Na x Co O 2 layered cobaltates

Measurements of polarization and temperature dependent soft x-ray absorption have been performed on ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$ single crystals with $x=0.4$ and $x=0.6$. They show a deviation of the local trigonal symmetry of the $\mathrm{Co}{\mathrm{O}}_{6}$ octahedra, which is temperature independent in a temperature range between $25\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $372\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This deviation was found to be different for ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ sites. With the help of a cluster calculation we are able to interpret the Co ${L}_{23}$-edge absorption spectrum and find a doping dependent energy splitting between the ${t}_{2g}$ and the ${e}_{g}$ levels $(10\phantom{\rule{0.3em}{0ex}}\mathrm{Dq})$ in ${\mathrm{Na}}_{x}\mathrm{Co}{\mathrm{O}}_{2}$.