Thermoelectric power of delafossite-derived compounds RCuO2+δ; (R = Y, La, Pr, Nd, Sm, and Eu)

The thermoelectric power of triangle planar cuprates, RCuO2+delta (delta greater than or equal to 0.5), of the delafossite-type with various rare-earth elements R: Y, La, Pr, Nd, Sm, and Eu has been studied. The thermoelectric power coefficient (S) was found to be positive for all the samples in the temperature range between 20 and 320 K. Contrary to the semiconducting behavior in the electrical conductivity, a small value of S less than or equal to 50 mu V/K was observed at room temperature. Except for the case of R = Pr, S of each sample decreased nonlinearly down to around 50 K and then increased rapidly as the temperature decreased, suggesting a phase transition or crossover to the insulating phase. For the case of PrCuO2+delta, on the other hand, S increased monotonically with decreasing temperature. Taking into account both the carrier-diffusion and the phonon-drag process, the nonlinear temperature dependence of S between 50 and 320 K was analyzed. In spite of rather small S in magnitude, which is comparable to that of metals and/or semimetals, the observed thermopower from hopping contributions may indicate that the sample is still in the nonmetallic state.