  Y$_{0.9}$Gd$_{0.1}$Fe$_{2}$, which crystallizes 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$_{2}$H$_{x}$ hydrides (2.9 $\le$ x $\le$ 5) crystallize
in three phases with cubic structure (C1, C2 and C3), two phases with
monoclinic structures (M1 and M2), and one phase with orthorhombic structure
(O), 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 M1 and M2 and the
cubic C2 phases undergo order-disorder (O-D) transitions toward a disordered
cubic structure CDis. These O-D transitions are reversible with thermal
hysteresis effects. The cubic C3 and orthorhombic O phases transform into a
disordered cubic phase accompanied by H desorption.
