The aim of the present work was to investigate the microstructure of bulk niobium irradiated by 10 MeV electrons. Positron-annihilation spectroscopy was employed as a principal technique for the characterization of irradiation-induced defects. Experimental results were compared to first-principles theoretical calculations of positron characteristics. In addition to extended positron-annihilation studies, the specimens were characterized also by X-ray diffraction. It was found that irradiation-induced vacancies were surrounded by hydrogen. Complexes consisting of a Nb vacancy surrounded by one and two H atoms were identified in the irradiated specimens. The concentration of these vacancy-hydrogen complexes was estimated to be (18–24) x 10−5 at. %. Vacancy-2H complexes were found to represent the dominating type of defects. Hydrogen atoms surrounding a Nb vacancy cause a shortening of the lifetime of trapped positrons. Moreover, it was demonstrated that hydrogen attached to Nb vacancy could be identified by coincidence Doppler broadening technique. The effect of a thin Pd (or Cr) overlayer on the irradiation-induced defects was investigated also. It was found that the relative fraction of vacancy-2H complexes was higher in the specimens irradiated with such an overlayer

Hydrogen-Vacancy Complexes in Electron-Irradiated Niobium. J.Čížek, I.Procházka, S.Daniš, G.Brauer, W.Anwand, R.Gemma, E.Nikitin, R.Kirchheim, A.Pundt, R.K.Islamgaliev: Physical Review B, 2009, 79[5], 054108