Correlation between the electronic structure and the thermoelectric properties of Gd-doped CaMnO_3-δ

We report x-ray absorption near edge structure (XANES) on bulk samples of Ca_1-x Gd _x MnO_3+δ with x = 0, 0.02, and 0.05. The transport properties measurements indicate that both resistivity and the absolute value of thermopower decrease with increasing x. Meanwhile, the temperature dependence of resistivity changes from non-metallic CaMnO _3-δ to metallic for samples of x = 0.02 and 0.05. XANES spectra at the O K-edge, Mn L _2,3-edge, and Ca L _2,3-edge indicate that Mn 3d-O 2p hybridization increases considerably, and unoccupied (3d-4s) states of Ca decrease with increasing substitution content. The increases of Mn 3d-O 2p hybridization might be responsible for the decrease of the resistivity and the absolute value of thermopower upon partial Gd substitution for Ca.