2004 Volume 45 Issue 2 Pages 292-295
We systematically investigated the hydrogenation and dehydrogenation properties for heavy rare earth-based binary RHNi5 (RH = Gd, Tb and Dy) intermetallic compounds and evaluated the correlations between crystallographic and thermodynamic properties. XRD analysis shows that all RHNi5 compounds crystallize in the hexagonal CaCu5-type crystal structure. In analogy to the light rare earth-based RLNi5 (RL = La, Pr, Nd and Sm) compounds both lattice constants of RHNi5 compounds decrease with increasing the atomic number of RH element due to the lanthanide contraction. On the pressure-composition (P-C) isotherms, GdNi5-H2 system shows two well-separated pressure plateaux qualitatively similar to RLNi5-H2 systems. Looking over from Gd to Dy in the RHNi5 compounds, we find three specific dehydrogenation properties on the P-C isotherms: 1. The first plateau pressure (pP1) increases in this order (at around H/RHNi5 = 2.5) due to less stability of hydrogen in the unit cell by the lanthanide contraction. Linear correlations are also observed between log pP1 and the unit cell volume (V) which fall onto the same lines extrapolated from those observed in case of the RLNi5 compounds. 2. The second plateau (P2) tends to disappear because the P-C isotherm goes beyond the critical point of the phase transition. 3. Fairly flat first plateau separates into two parts in which a new plateau (PN) appears at low hydrogen content (H/RHNi5 ≤ 2) with hysteretic phase transition. So long as the first plateau of dehydrogenation is concerned, from LaNi5 to DyNi5 we can predict the first plateau pressure from the unit cell volume of compounds and temperature.