2004 Volume 45 Issue 8 Pages 2610-2613
Pressure-composition (P-C) isotherms of RNi5-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 (α phase; RNi5H∼0.5) and two hydrides (β phase; RNi5H3∼4 and γ phase; RNi5H5∼7). To explore the mechanism of the phase transitions, we investigated the structural changes in RNi5-H systems with ex-situ XRD. During hydrogen desorption, the ex-situ XRD profiles show that the crystal structures in RNi5-H systems change from hexagonal (γ phase) through monoclinic (β phase) to hexagonal (α 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 α and β phases normalized in the monoclinic structure decreases slightly with the increasing atomic number of R in RNi5-H systems. Similarly to the correlation between the first plateau pressure and the unit cell volume of the α phase, the second plateau pressure is logarithmically related to the unit cell volume of the β phase. As far as the RNi5-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.
