X-ray absorption spectroscopy of NaxCoO2 layered cobaltates

Measurements of polarization and temperature dependent soft x-ray absorption have been performed on NaxCoO2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co3+ and Co4+ 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 Dq) in NaxCoO2.