Hydrogenation and Dehydrogenation Properties of R<SUB>H</SUB>Ni<SUB>5</SUB> (R<SUB>H</SUB> = Heavy Rare Earth) Binary Intermetallic Compounds

Abstract

We systematically investigated the hydrogenation and dehydrogenation properties for heavy rare earth-based binary R H Ni 5 (R H = Gd, Tb and Dy) intermetallic compounds and evaluated the correlations between crystallographic and thermodynamic properties. XRD analysis shows that all R H Ni 5 compounds crystallize in the hexagonal CaCu 5 -type crystal structure. In analogy to the light rare earth-based R L Ni 5 (R L = La, Pr, Nd and Sm) compounds both lattice constants of R H Ni 5 compounds decrease with increasing the atomic number of R H element due to the lanthanide contraction. On the pressure-composition (P-C) isotherms, GdNi 5 -H 2 system shows two well-separated pressure plateaux qualitatively similar to R L Ni 5 -H 2 systems. Looking over from Gd to Dy in the R H Ni 5 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 H Ni 5 = 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 L Ni 5 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 H Ni 5 2) with hysteretic phase transition. So long as the first plateau of dehydrogenation is concerned, from LaNi 5 to DyNi 5 we can predict the first plateau pressure from the unit cell volume of compounds and temperature.

Structure–properties relationship in RE3−Mg Ni9H10–13 (RE = La,Pr,Nd) hydrides for energy storage

“…A similar correlation between the unit cell volumes of the intermetallic alloys and thermodynamic stability of the hydrides is a well-documented feature for the RENi 5 H x hydrides (see e.g. [13]) and was also recently observed for the series of hydrogenated Mg-containing La 3-x Mg x Ni 9 (x=0-2) intermetallic alloys [5]. Changing of the Mg/La ratio in the La 3−x Mg x Ni 9 compounds allows one to achieve a huge variation of the hydride thermal stabilities.…”

“…Impurities give a minor contribution to the overall hydrogenation performance. Indeed, PrNi 5 and NdNi 5 remain nonhydrogenated as they absorb hydrogen at pressures exceeding 25 bar (room temperature) [13] which is above the pressure range used in the present work. On the other hand, at P eq ≈1 bar at 50°C REMgNi 4 form REMgNiH 4 hydrides containing appr.…”

Appearance of a Novel Pressure Plateau in RNi<SUB>5</SUB>-H (R = Rare Earth) Systems

“…1. The constants of HoNi 5 compound fall onto the same line extrapolated from observations of other RNi 5 compounds, 6) and those of YNi 5 compound fall between those of TbNi 5 and DyNi 5 compounds.…”

“…This suggests the presence of a single HoNi 5 hydride. Another measurement of P-C isotherms at 196 K shows that a second pressure plateau similar to that found in TbNi 5 -H and DyNi 5 -H systems 6) is minimally observable. The maximum hydrogen content in this system is H/HoNi 5 ¼ 2:0 at 248 K, which is smaller than that of ternary HoNi 5Àx M x compounds (M ¼ Al, Gd; x ¼ 0:5, 1.0).…”

“…We found that the first (lower) plateau separates to show a novel pressure plateau at low hydrogen content, indicating a novel phase transition. 6) Nevertheless, it remains unclear why the novel plateau appears. The correlation between the novel plateau and the R element in RNi 5 -H systems is not currently known.…”