Hydrogen isotherms for annealed, un-activated LaNi5 (273–333 K)

•Isotherms for un-activated LaNi5 down to 273 K.•ΔH for hydride formation less exothermic for un-activated than for activated LaNi5.•Absorption isotherms are sloping for un-activated LaNi5 whereas they are nearly horizontal for desorption of the same sample.•Sloping of un-activated LaNi5 absorption isotherm due to internal stresses since desorption for same sample is nearly horizontal.

Hydrogen absorption isotherms (273–333 K) have been measured for closely stoichiometric, annealed, un-activated LaNi5, i.e., a virgin un-activated LaNi5 sample was employed at each temperature. This is the first time that such isotherms have been measured for annealed, un-activated LaNi5 at such low temperatures ⩽298 K. After measurement of the initial absorption isotherms, the second absorption and desorption isotherms have been measured at each temperature. Van’t Hoff plots have been made from these absorption and desorption plateau pressure data and, from these, ΔHplatΔHplat values have been obtained for the initial and subsequent absorption and desorption reactions. The ∣ΔHplat∣ determined from the van’t Hoff plot is smaller for the initial isotherm for the annealed, un-activated form of LaNi5 than it is for activated LaNi5.