Structural Changes in RNi<SUB>5</SUB>-H (R = Pr, Nd, Sm and Gd) Systems with Two Hydrogen Pressure Plateaux

Abstract

Pressure-composition (P-C) isotherms of RNi 5 -H (R = Pr, Nd, Sm and Gd) systems show two hydrogen pressure plateaux during hydrogen absorption and desorption. These correspond to the transitions among three phases: one hydrogen solid solution ( phase; RNi 5 H $0:5 ) and two hydrides ( phase; RNi 5 H 3$4 and phase; RNi 5 H 5$7 ). To explore the mechanism of the phase transitions, we investigated the structural changes in RNi 5 -H systems with ex-situ XRD. During hydrogen desorption, the ex-situ XRD profiles show that the crystal structures in RNi 5 -H systems change from hexagonal ( phase) through monoclinic ( phase) to hexagonal ( phase). The temporary decrease in structural symmetry may be due to hydrogen occupation except in the basal plane of the crystal structure. Lattice expansion between the and phases normalized in the monoclinic structure decreases slightly with the increasing atomic number of R in RNi 5 -H systems. Similarly to the correlation between the first plateau pressure and the unit cell volume of the phase, the second plateau pressure is logarithmically related to the unit cell volume of the phase. As far as the RNi 5 -H (R: from La to Gd except Ce) system is concerned, we can empirically predict the second plateau pressure as well as the first plateau pressure from the unit cell volume of the alloys.

A theoretical interpretation of the pressure–composition isotherms of RNi5 (, Pr, Nd and Sm) systems based on statistical mechanics

“…The modified space group of full hydride phase is not P6/mmm but P63mc [22,23]. Previously we have found from an ex situ XRD analysis that RNi 5 (R = Pr, Nd and Sm) systems underwent the same phase change from P6/mmm (˛phase) to P63mc ( phase) as in the case of LaNi 5 [24]. In addition, the experimentally determined PCIs in Fig.…”

“…The average and the twice of the standard deviation are (kJ mol −1 ) = −3.81 ± 0.34. The constancy of the relaxation parameter can be explained by the fact that RNi 5 -H (R = Pr, Nd and Sm) systems undergo similar structural changes during the hydrogen desorption [24]. The PCIs obtained by fixing and re-optimizing other parameters are drawn with lines in Fig.…”

“…In the formula unit of AB 5 structure, there are 37 potential sites available as interstitials; 3f (A 2 B 4 ), 12n (AB 3 ) near the basal plane (z = 0) and 6m (A 2 B 2 ), 12o (AB 3 ) and 4h (B 4 ) close to the z = 1/2 plane in terms of Wyckoff notation [17]. Interstitial sites occupied by hydrogen or deuterium atoms in LaNi 5 -H system have been investigated by X-ray and neutron diffractometry as well as by calculation [18][19][20][21][22][23][24]. Wastlake's two geometrical criteria -the minimum hole size and H-H distance should be, respectively, 0.4 and 2.1Å -imply that the hydrogen distribution in LaNi 5 H 7 would be four in 3f + 12n + 12o, two in 6m and one in 4h [21].…”

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

“…In the RNi 5 -H system, two hydrogen sites correspond to the lower and higher plateaux. 15,16) The novel plateau may be attributed to another site whose size is reversed by the unit cell volume. To determine the size of the hydrogen site, it is necessary to clarify the structure of novel RNi 5 hydrides such as HoNi 5 hydride, with specific reference to the novel plateau.…”

Thermodynamical Modeling of P-C Isotherms for Metal Hydride Materials

“…They both show a doubled unit cell along the c-axis. In two recent studies, the authors concluded that RNi 5 compounds have the same crystal structure (P6/mmm), the same phase change and similar hydrogenation properties [25,26]. Here, we adopt the postulation made by Westlake about the maximum occupancy at each site in a formula unit, and it is given in Table I.…”