Site Occupancy of Hydrogen in a Distorted bcc Lattice in Niobium Alloyed with Mo Atoms

Abstract

In order to study the atomistic state of hydrogen in a bcc lattice, the site occupancies of hydrogen dissolved in Nb–Mo alloys containing 39, 48, and 60 at. % Mo atoms have been investigated at room temperature by the channelling method utilizing a nuclear reaction of 1 H( 11 B,α)αα with a 11 B + beam of about 2 MeV. In addition, the lattice parameters and half-widths of the X-ray reflection lines of the non-hydrogen-doped specimens have been measured at room temperature. Nb–Mo alloys form a solid solution over the entire Mo concentration range, maintaining a bcc crystal structure. From systematic studies including previous experiments on such alloys with lower Mo concentrations, it has been observed that the site occupancy of hydrogen changes very sensitively with Mo concentration. At low Mo concentrations, hydrogen is trapped by Mo atoms and occupies the trapped site ( T tr ), which is displaced by about 0.6 Å from a tetrahedral ( T ) site attached to a Mo atom towards that Mo atom. At approximately 20 at. % Mo, most of the H atoms occupy T sites and some portion of the H atoms occupy octahedral ( O ) sites. With increasing Mo concentration, the O -site occupancy diminishes, and at Mo concentrations higher than about 39 at. % the occupancy of the displaced- T (d- T ) site newly appears, which is displaced by about 0.25 Å from a T site towards its nearest-neighbour O site. It is recognized that there exists a close correlation between such a site change and a change in the half-width of the X-ray reflection line with Mo concentration. On the basis of this correlation, it is concluded that the change in the site occupancy of hydrogen is due to a change in the lattice distortion induced by alloying with undersized Mo atoms, and that the d- T site is a stable site for hydrogen in slightly distorted bcc lattices.