High-Temperature Thermoelectric Power of The Metal Oxides: La2− x Sr x CuO4 and Bi1− x Sr x MnO3

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We have investigated the high-temperature thermoelectric power (TEP) of La2−xSrxCuO4 (0.05 ≤ x ≤ 0.35) and Bi1−xSrxMnO3 (0.5 ≤ x ≤ 0.8) up to 700 K. Based on the TEP results we have discussed the phase transitions on each case. In the case of high-TC cuprates, La2−xSrxCuO4 (0.05 ≤ x ≤ 0.35), the TEP shows different temperature dependences in three temperature regions. At low temperature, the positive TEP rises showing a broad peak at temperature TP, which shifts to lower temperature upon Sr doping. Right above TP ,t he TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, TH, showing a saturation behavior. The systematic change of the TEP behavior is discussed in terms of the two-fluids model, which is an intrinsically inhomogeneous state, consisted of bound pairs and independent carriers in the normal state of the high-TC superconductors. For Bi1−xSrxMnO3 (0.5 ≤ x ≤ 0.8), the negative TEP is almost temperature-independent in the high temperature regime (TCO < T < 700 K). Near the charge ordering temperature (TCO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, TCO shifts to lower temperature from TCO ∼ 520 K for x = 0.5 to TCO ∼ 435 Kf orx = 0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s 2 character of Bi 3+ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion.