Anomalous lattice expansion of metal-hydrogen thin films

We report x-ray diffraction experiments investigating the lattice expansion due to hydrogen in Nb/Ta superlattices and thin Nb films. For low hydrogen concentrations (c_H < 0.09 H/metal), the lattice expands only normal to the plane of the film due to the epitaxial constraints imposed by the substrate. At higher concentrations, corresponding to strains on the order of the critical strains expected for the formation of misfit dislocations, we find a structural instability which leads to the formation of two metal-H phases, one of which has a lattice expansion in the plane of the film. These results differ significantly from the expected phase diagram and properties of the bulk metal-H systems.