  We have proposed a qualitative model for the structure of binary systems
similar to Pd-Er alloys, which explains their nonmonotonic relaxation after the
hydrogen saturation. It is based on the assumption that such a solid solution
involves two kind heterogeneities. The former are caused by spinodal
decomposition of the initially homogeneous state of the solid solution into the
phases enriched and depleted of Er atoms. The latter are crystalline defects
that trap an additional amount of Er atoms, which leads also to their local
accumulation, changing the defect properties. Hydrogen atoms penetrating into
the solid disturb the equilibrium of both the phase separation and the defect
saturation with Er atoms, causing redistribution of Er atoms. The diffusion
fluxes give rise to the motion of the interface between the two phases that is
responsible for time variations, for example, in the relative volume of the
enriched phase observed experimentally. We have found the conditions when the
interface motion can change the direction during the system relaxation to a new
equilibrium state. The latter effect is, from our point of view, the essence of
the hydrogen induced nonmonotonic relaxation observed in such systems. The
numerical simulation confirms the basic assumptions.
