High-temperature thermoelectric power of the metal oxides: La2- x Sr x CuO4 and Bi1- x Sr x MnO3

We have investigated the high-temperature thermoelectric power (TEP) of La_{2- x} Sr _x CuO₄ (0.05≤x≤0.35) and Bi_{1- x} Sr _x MnO₃ (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-T _C cuprates, La_{2- x} Sr _x CuO₄ (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 T _P, which shifts to lower temperature upon Sr doping. Right above T _P, the TEP decreases linearly as temperature increases. At high temperature, TEP deviates from the linear-T dependence at a certain temperature, T _H, 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-T _C superconductors. For Bi_{1- x} Sr _x MnO ₃ (0.5≤x≤0.8), the negative TEP is almost temperature- independent in the high temperature regime (T _CO≤T≤700 K). Near the charge ordering temperature (T _CO), however, TEP suddenly decreases with decrease of temperature, indicating the suppression of carrier mobility with charge ordering transition. As Bi concentration decreases, T _CO shifts to lower temperature from T _CO∼520 K for x=0.5 to T _CO∼435. K for x=0.8, which suggests that charge ordering is related to the local lattice distortion due to highly polarizable 6s² character of Bi³⁺ ion. In comparison with the resistivity data, the TEP results have been discussed in terms of the carrier localization accompanied by local lattice distortion. © 2005 Springer Science+Business Media, Inc.