Dynamics of H in a Thin Gd Film: Evidence of Spinodal Decomposition

Gd thin films react at room temperature with hydrogen to form hydrides, by nucleation and growth, even for very low H content (H/Gd > 0.01). This phase transformation can be destabilized and suppressed in highly stressed films. In the present study, a thin Gd layer was deposited on a W(110) substrate, leading to a highly strained film. Following exposure to hydrogen, the overall strain in the film is further increased. Hydrogen was found to dissolve in the metallic matrix without forming distinct hydride nuclei. However, the lateral distribution of H in the film evolved with time, from a rather homogeneous repartition to an inhomogeneous one, reflecting the process of spinodal decomposition of hydrogen in the film. The spinodal decomposition process was monitored using scanning tunneling microscopy. This process involves principally the diffusion of H in the film, but a slow change in shape of the Gd islands covering the wetting layer was also observed. These changes were used to monitor the evolution of the local strains and hydrogen concentrations with time and to draw strain and composition maps at different times, before and after hydrogenation. Numerical simulations of the process, using the chemical potential of H in the highly strained film and applying the Cahn-Hilliard equation, were shown to be in good agreement with the experimental observations, in both spatial and temporal scales. The present study shows that high tensile strains strongly affect the dynamics of the H distribution and the composition of the H-containing phases, opening the route for future studies of M-H systems on the nanometer scale.