Transport properties of Ce-doped RMnO3 (R = La, Pr, and Nd) manganites

in Ce-doped RMnO3 the rare: earth (R) is partially replaced with cerium ions instead of divalent alkaline-earth metals. These compounds are very sensitive to annealing. The resistivity and thermoelectric power (TEP, S) of R0.7Ce0.3MnO3 (R = La, Pr, and Nd) have been studied as a function of annealing conditions. They show a ferromagnetic to paramagnetic transition, and a resistive peak near T-c, marking a metal-insulator (MI) transition. A pronounced giant magnetoresistance effect is also observed in these compounds, and T-c can be changed by annealing in various atmospheres. The rho(T) curve of oxygen overdoped samples shows double peaks slang with a hysteresis loop (in the heating and cooling cycles), which bears the signature of the first-order nature of the transition. The thermopower of as-prepared samples show a peak near the MI transition, and changes sign from positive to negative at temperatures below the ferromagnetic transition. The magnitude of S at low temperatures is enhanced to an abnormally large value. With increasing of oxygen content. several characteristic features develop in the temperature dependence of the thermopower which is consistent with the resistive behavior. At high temperatures, the resistivity and thermopower follow the predictions of the Emin-Holstein theory of adiabatic polaron hopping.