Analysis of phase transformation processes observed in hydrogen absorbing materials
(pure metals, alloys, or compounds) is still a matter of active research. Using pneumato-chemical
impedance spectroscopy (PIS), it is now possible to analyze the mechanism of hydriding
reactions induced by the gas phase. Experimental impedance diagrams, measured on activated
LaNi5 in single- and two-phase domains, are reported in this paper. It is shown that their shape is
mostly affected by the slope of the isotherm at the measurement point. By considering the details
of the multistep reaction paths involved in the hydriding reaction, model impedance equations
have been derived for single- and two-phase domains, and fitted to experimental impedance
diagrams. The possibility of separately measuring surface and phase transformation resistances,
hydrogen diffusion coefficient, and hydrogen solubility in each composition domain is discussed.