Hydrogenation and Dehydrogenation Properties of R_HNi₅ (R_H = Heavy Rare Earth) Binary Intermetallic Compounds

Abstract

We systematically investigated the hydrogenation and dehydrogenation properties for heavy rare earth-based binary R_HNi₅ (R_H = Gd, Tb and Dy) intermetallic compounds and evaluated the correlations between crystallographic and thermodynamic properties. XRD analysis shows that all R_HNi₅ compounds crystallize in the hexagonal CaCu₅-type crystal structure. In analogy to the light rare earth-based R_LNi₅ (R_L = La, Pr, Nd and Sm) compounds both lattice constants of R_HNi₅ compounds decrease with increasing the atomic number of R_H 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 R_LNi₅-H₂ systems. Looking over from Gd to Dy in the R_HNi₅ compounds, we find three specific dehydrogenation properties on the P-C isotherms: 1. The first plateau pressure (p_P1) increases in this order (at around H/R_HNi₅ = 2.5) due to less stability of hydrogen in the unit cell by the lanthanide contraction. Linear correlations are also observed between log p_P1 and the unit cell volume (V) which fall onto the same lines extrapolated from those observed in case of the R_LNi₅ 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/R_HNi₅ ≤ 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.