Compositional properties and thermoelectric power of the superconducting ceramic Nd_2-xCe_xCuO_4-y

We report studies on the compositional properties and subsequent thermoelectric power of ceramic samples of superconducting reduced, Ce-doped Nd2CuO4-y and for the undoped parent compound as well. The synthesis of ceramic material with uniform cerium concentration is difficult, but samples can be obtained which are 20-25% superconducting by volume. For such material we find the sign of the Seebeck coefficient to be negative at all temperatures above its superconducting transition at T = 22 K and zero below, consistent with, but not proving purely "n-type" normal state transport and pairing. We also obtain a negative room temperature thermopower for the parent Nd2CuO4-y compound, but with a change to positive sign at lower temperature. We review these findings against thermopower data reported for the "p-type" high-Tc compounds where a negative Seebeck coefficient is also observed under certain conditions, and conclude that separation of carriers into specific negative or positive entities in the spirit of the effective mass approximation is in general not possible in copper oxide perovskite superconductors. © 1989.