Solubility of hydrogen in thin niobium films

The resonant nuclear reaction 1H(15N,αγ)12C was used to measure hydrogen concentrations in thin Nb films as a function of the external H2 pressure (H solubility). Experiments were performed in the pressure range up to 1 mbar and temperatures between 166 and 200 degree C, i.e., in the region around the critical temperature (177 degree C) of the α-α' phase transition. We find that the hydrogen solubility in thin epitaxial films is considerably reduced compared to bulk data. The analysis of the data within mean-field theory for a lattice gas shows that the H-Nb interaction remains, within the experimental error, unchanged with respect to the bulk value but that the attractive H-H interaction energy is strongly reduced (0.1 eV in the film compared to 0.2 eV in the bulk). The difference is attributed to strain effects due to the clamping of the films to the substrate