X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2
  Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and 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 (10Dq) in Na_xCoO_2. 
T. Kroll, M. Knupfer, J. Geck, C. Hess, T. Schwieger, G. Krabbes, C.   Sekar, D.R. Batchelor, H. Berger, and B. Buechner