Electrical conductivity of uranium-neptunium mixed oxides

The electrical conductivities (sigma) of NpyU1-yO2+x (y=0.1 and 0.2) were measured in the range of 10(-15)less than or equal to P-O2 (Pa)less than or equal to 10(-1.5) at 1273 K and in the temperature range from room temperature to 1273 K for the specimen equilibrated at constant P-O2 by the four inserted wires method. Four different dependencies of sigma upon P-O2 for NPyU1-yO2+x were observed, similar to the results for UO2+x. Two types of curve were obtained in the log(sigma) vs. 1/T plots for NpyU1-yO2+x, i.e. L- and Z-shaped curves. Specimens equilibrated at P-O2 below 10(-4.85) Pa showed an L-shaped curve, whereas those equilibrated at P-O2 between 10(-4.85) and 10(-2.7) Pa showed a Z-shaped one. The activation energy of the electrical conduction for the samples with Z-shaped curves was about 0.3 eV, which was the same value as that of UO2+x in the same P-O2 region. From these observations, it is suggested that Np exists as a Np4+ ion in these solid solutions and plays no important role on the electrical conduction. (C) 1998 Elsevier Science S.A.