Electrical Properties of the Cerium and Gadolinium Hydrogen Systems

Conductivities of polycrystalline solid samples of cerium hydride and gadolinium hydride were measured over the composition ranges H/Ce=0–2.5 and H/Gd=0–2.13 at room temperature and at elevated temperatures (400°—750°C). Conduction is metallic and the results for the two systems are very similar. The conductivity in the variable composition dihydride phase of each system decreases with increasing hydrogen content and is explained assuming that the hydrogen is anionic, taking its extra electron from the system's conduction band. Measurements of absolute Seebeck coefficients (CeH2.37, —17 μV/°C; GdH2.12, —6 μV/°C) support this model. The measured conductivity curves extrapolate to zero at CeH2.7 and GdH2.3. For the dihydride phases the composition dependencies of the conductivities are used to calculate carrier mobilities. The values obtained are in the range 1 to 10 cm2/V-sec. The conductivities of compositions H/M=0 to 2 show that the dihydrides can exist with hydrogen-to-metal ratios much less than 2.0. Ionic conduction has been found not to be important to the conduction mechanism from the results of a concentration cell measurement.