SOLUBILITY OF HYDROGEN IN THIN NIOBIUM FILMS

The resonant nuclear reaction H-1(N-15,alphagamma)C-12 was used to mea sure hydrogen concentrations in thin Nb films as a function of the ext ernal H-2 pressure (H solubility). Experiments were performed in the p ressure range up to 1 mbar and temperatures between 166 and 200-degree s-C, i.e., in the region around the critical temperature (177-degrees- C) of the alpha-alpha' phase transition. We find that the hydrogen sol ubility in thin epitaxial films is considerably reduced compared to bu lk data. The analysis of the data within mean-field theory for a latti ce gas shows that the H-Nb interaction remains, within the experimenta l error, unchanged with respect to the bulk value but that the attract ive H-H interaction energy is strongly reduced (0.1 eV in the film com pared to 0.2 eV in the bulk). The difference is attributed to strain e ffects due to the clamping of the films to the substrate.