Site occupancy change of hydrogen in Nb-Mo alloys with Mo concentration

The site occupancy of hydrogen in Nb(74 at. %)-Mo(26 at. %) alloys is investigated at room temperature for hydrogen concentrations 0.016, 0.026, and 0.059 in the hydrogen-metal-atom ratio ([H]/[M]) by the channeling method utilizing a nuclear reaction H-1(B-11,alpha)alphaalpha with a B-11 beam. For comparison, the site occupancy of hydrogen in Nb is also investigated for [H]/[M]=0.023. The channeling angular profiles obtained for the alloys are very similar to those for the Nb specimen. It is concluded that, irrespective of the hydrogen concentration, more than 90% of H atoms are located at tetrahedral (T) sites in the Nb(74 at. %)-Mo(26 at. %) alloys. This is different from the previous result on H in Nb(80 at. %)-Mo(20 at. %) alloys, in which at low hydrogen concentrations the O-site occupancy of 20%-30% of H atoms was observed in addition to the T-site occupancy. The present results on the alloys with 26 at. % Mo indicate that, even if H atoms are located at O sites, their fraction is at most 10%, irrespective of the hydrogen concentration. A change in the site occupancy of hydrogen in Nb-Mo alloys with Mo concentration is discussed, including results previously obtained for alloys with lower concentrations of Mo atoms.