Phase relations and electrical conductivities in the Nd-Ce-Cu-O system

A systematic study by means of X-ray diffraction, differential thermal analysis coupled with thermogravimetry, scanning electron microscopy-energy-dispersive X-ray probe and high temperature resistivity of samples of the Nd-Ce-Cu-O system allowed us to determine precise phase relations in the binary NdO1.5-CuO and CeO2-CuO and ternary NdO1.5-CeO2-CuO systems. The ability of cerium to form mixed valence compounds, complemented by the reaction 2CuO → Cu2O + 12O2, introduces an additional difficulty for the preparation or the high temperature heat treatment of well-defined samples. In cerium-doped specimens (Nd1−xCex)2CuO4−δ the limit of solubility was found to be x = 0.08. The solid solution is formed by non-congruent reaction at 1285 °C from an Nd2O3 + CeO2 + liquid mixture. We argue that a long-range ordering may influence the electronic properties at low temperature, especially the superconductivity.