-Hydrogen-induced defects in niobium studied by positron annihilation spectroscopy

Our aim in the present work was to investigate changes of the defect structure of bulk niobium induced by hydrogen loading. The evolution of the microstructure with increasing hydrogen concentration was studied by x-ray diffraction and two complementary techniques of positron annihilation spectroscopies (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with the measurement of the Doppler broadening, in defect-free Nb (99.9%) and Nb containing a remarkable number of dislocations. These samples were electrochemically loaded with hydrogen up to xH=0.06 [H/Nb], i.e. in the alpha-phase region, and it was found that the defect density increases with hydrogen concentration in both Nb samples. This means that hydrogen-induced defects are created in the Nb samples. A comparison of PAS results with theoretical calculations revealed that vacancy-hydrogen complexes are introduced into the samples due to hydrogen loading. Most probably these are vacancies surrounded by 4 hydrogen atoms.