Thermoelectric Properties and Local Electronic Structure of Rare Earth-doped Ca₃Co₂O₆

Thermoelectric properties of a series of rare earth metal-doped polycrystalline samples of (Ca_1-xR_x)₃Co ₂O₆ with R = Gd, Tb, Dy and Ho (x = 0 - 0.1) were investigated in the temperature range from 300 K to 1300 K. In a high temperature region above 900 K, a partial rare earth substitution with R ³⁺ for Ca²⁺ resulted in appreciable increase in the Seebeck coefficient (S). However, the S value decreased abruptly at low temperatures, and turned to negative values for the Gd- and Tb-doped samples at temperatures below 400 K. With decreasing ionic radii of rare earth elements (Gd³⁺ > Tb³⁺ > Dy³⁺ > Ho³⁺), the S values increased, while the thermal conductivity (kappa) decreased particularly at temperatures above 700 K. Contrastingly, the influence of rare earth metal substitution on the electrical resistivity (rho) was small; in high temperature region the rho values increased only slightly with decreasing ionic radii of rare earth metals. High-temperature thermoelectric figure-of-merit (Z) of the samples was thereby improved by the late rare-earth metal substitution for Ca²⁺, particularly for those with Ho³⁺. A maximum Z value of the Ho-doped sample for x = 0.03 was 1.83 times 10^-4 K^-1 at 1100 K as compared with 0.37 times 10^-4 K^-1 for non-doped sample. The electronic structure of the samples was also investigated by x-ray photoemission spectroscopy (XPS) technique. The charge-transfer satellite structure of Co 2p core-level spectra was observed for the Gd-and Tb-doped samples, while the satellite is negligible for the other samples.