STABILITY OF THE HYDROGEN ABSORPTION AND DESORPTION PLATEAUS IN LANI5-H .1. HYSTERESIS DYNAMICS AND LOCATION OF THE EQUILIBRIUM ISOTHERM

Several aspects of pressure hysteresis in LaNi5Hx have been experimentally investigated by the aliquot method. These aspects include the important question of the existence of a unique equilibrium absorption or desorption isotherm, and the supposed ''large-aliquot'' effect, which will be dealt with in a subsequent paper. In this paper, a systematic survey of hysteresis as a function of the measurement time and step length on the plateaux is reported. We found that there is a minimum achievable hysteresis, even after very long measurement times (such as 10 days for one hysteresis loop); therefore, we conclude that a unique isotherm does not exist. We found evidence to suggest that neither plateau should be regarded as being closer to equilibrium than the other. Instead, the plateaux appear to be near equilibria in two thermodynamically distinct systems.