X-ray absorption spectroscopy studies of Ca _2.9Ln _0.1Co ₄O _9+δ (Ln = Ca, Dy, Ho, Er and Lu)

X-ray absorption near edge structure (XANES) study of Co K-, Co L _2,3-, O K-, and Ca L _2,3-edges on a series of polycrystalline Ca _2.9Ln _0.1Co ₄O _9+δ (Ln = Ca, Dy, Ho, Er and Lu) are presented. The high similarity of Co K-edge spectra of the doped and the undoped samples indicate that the average valence of Co is slightly higher than 3+ and it is not varied for different dopants. The variation of the intensity of the Co L _2,3-edge indicates that the number of Co 3d unoccupied states and thermoelectric power shows an opposite trend. According to the O K-edge spectra, the electrical resistivity and Co ⁴⁺ 3d-O 2p hybridized unoccupied states also show an opposite trend, which is consistent with our results that a lesser amount of Co ⁴⁺ leads to a larger thermoelectric power. There is a monotonic decrease of Ca L _2,3-edge intensity with decreasing ionic radius of the Ln element, which partially substituted for Ca. This could be attributed to a decrease of Ca 3d-O 2p hybridization with decreasing ionic radius of Ln.