The resonant nuclear reaction 'H("N, ay)' C was used to measure hydrogen concentrations in thin Nb films as a function of the external H~pressure (H solubility). Experiments were performed in the pressure range up to 1 mbar and temperatures between 166 and 200 C, i.e. , in the region around the critical temperature (177'C) of the a-a 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.

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c-axis-oriented YBa,Cu307 (1:2:3)and YBa2Cu408 (1:2:4) thin films were charged with hydrogen at 463 K and a H2 pressure of 100 mbar. The hydrogen concentrations and depth profiles were measured with the ' N nuclear reaction method. In contrast to earlier studies on bulk samples where a constant T, ,"", is reported, we find in our 1:2:3films that the superconducting transition temperature decreases continuously with increasing hydrogen concentration and that the transitions remain relatively sharp, e.g. , T, ,"", =38 K and b, T, =6.2 K for [H]/cell -=0.6. The electrical resistivity and Hall-effect measurements in the normal state indicate that the main effect of hydrogen doping is a reduction of the charge carrier concentration, whereas the carrier mobility is not changed significantly. At an average hydrogen concentration of [H]/cell =-2 in both systems, a new hydride phase is observed, which for 1:2:3is characterized by a c-axis expansion of 16% and for 1:2:4by a c-axis expansion of 1.5%.

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The lattice sites of implanted Li in GaAs single crystals have been determined between 80 and 300 K by measuring channeling and blocking effects of a particles emitted in the decay of implanted radioactive 8 Li. Below 220 K the tetrahedral interstital site is preferentially occupied, whereas between 220 and 300 K substitutional and irregular sites are increasingly populated. These site changes are attributed to, respectively, reactions with negatively charged Ga vacancies and defect complexes in recovery stages I and II of GaAs.