THERMOELECTRIC POWER OF DELAFOSSITE-DERIVED COMPOUNDS, RCUO2+DELTA (R =Y, LA, PR, ND, SM, AND EU)

The thermoelectric power of triangle planar cuprates, $R{\mathrm{CuO}}_{2+\mathrm{\ensuremath{\delta}}}$ $(\ensuremath{\delta}g~0.5),$ 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 $Sl~50\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ was observed at room temperature. Except for the case of $R=\mathrm{Pr},$ $S$ 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 ${\mathrm{PrCuO}}_{2+\mathrm{\ensuremath{\delta}}},$ 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.