Abstract
Y_(0.9)Gd_(0.1)Fe_2, which crystallize in a C15 cubic structure, can absorb up to 5 H/f.u. and its pressure-composition isotherm displays a multiplateau behavior related to the existence of several hydrides with different crystal structures. At room temperature Y_(0.9)Gd_(0.1)Fe_2H_x hydrides (2.9 ≤x ≤ 5) crystallize in three phases with cubic structure (C1, C2 and C3), two phases with monoclinic structures (Ml and M2), and one phase with orthorhombic structure (0), with the following sequence for increasing H concentration: C1, M1, C2, M2, C3, O. Each phase exists as single phase within a H homogeneity range, and they are separated from each other by two-phase domains. The reductions of crystal symmetry are related to various hydrogen orders into interstitial sites. Weak superstructure peaks were indexed by doubling the cubic cell parameter of the cubic C2 phase. Upon heating, the monoclinic Ml and M2 and the cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered cubic structure C_(Dis). These O-D transitions are reversible with thermal hysteresis effects. The cubic C3 and orthorhombic 0 phases transform into a disordered cubic phase accompanied by H desorption.
NSTL
Elsevier