Hysteresis effects in cerium-containing LaNi5-type compounds

The (La0.5Ce0.5)Ni5−xCox-hydrogen system was studied using pressure-composition and X-ray diffraction techniques. The (La,Ce)Ni5 compounds display very large hysteresis effects in the course of a hydrogénation cycle. The addition of cobalt markedly affects the absorption plateau pressure which reverts to normal values at x = 1, practically eliminating the hysteresis effect. The results indicate that the elevated absorption plateau pressure and the resulting hysteresis effect are due (1) to the presence of cerium atoms and (2) to the formation of the expanded HI LaNi5-type hydride. The formation of this greatly expanded hydride phase favours the reversion of cerium to the trivalent state. The substitution of 20% of the nickel atoms by cobalt atoms suppresses the formation of the HI hydride which is replaced by the less expanded HII hydride. The HII hydride does not display anomalous hysteresis effects.