Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 50 (2017)

㻌Site occupancy of hydrogen in Ta-rich Ta-Nb alloys as observed by the channelling
methodY. Okada,㻖1, 㻖2㻌E. Yagi,㻖1, 㻖2㻌H. Matsuba,㻖2 C. Sugi,㻖2 S. Koike,㻖3 S. Nakamura,㻖1, 㻖2㻌T. Sugawara,㻖4 T. Shishido,㻖4and K. Ogiwara㻖1In metal-hydrogen systems, alloying with substitutional
or interstitial solutes has significant effects on varioushydrogen properties in these systems, e.g., the terminal
solubility of hydrogen, hydrogen diffusivity etc. One of theimportant factors controlling such effects is hydrogen-soluteinteraction. As an example for substitutional solutes, theinteraction in the Nb-based Nb-Mo and Nb-Ta alloys hashitherto been studied systematically in the soluteconcentration range up to 60 at. % through the observationof a change in the lattice location of hydrogen. Informationon the lattice location of hydrogen had been extremelylimited, because of experimental difficulties. Therefore, thechannelling method utilizing a nuclear reaction of1H(11B,D)DD with a 11B+ beam of about 2 MeV with aTandem accelerator was developed.1) Hydrogen can bedetected by measuring emitted D particles.
Both Nb-based Nb-Mo and Nb-Ta alloy systems form asolid solution over the entire solute concentration (CMo andCTa) range, maintaining a bcc crystal structure, although thelattice parameter changes. Their atomic radii are 1.43 Å forNb, 1.36 Å for Mo (undersized solute) and 1.44 Å for Ta(slightly oversized solute). It has been demonstrated that, inthe Nb-Mo alloys, the lattice location of hydrogen changes
very sensitively to CMo, whereas, in the Nb-Ta alloys, theCTa dependence of the site change is very weak. With thehelp of measurement of X-ray reflection line width, such achange was interpreted on the basis of lattice distortion
induced by alloying.2) Broadening of reflection lines serves
as a measure of lattice distortion averaged over the wholespecimen. Hydrogen is located at tetrahedral (T) sites in Nband Ta.
In the Nb-Mo alloys, at low CMo, lattice is strongly
distorted around individual undersized Mo atoms.
Hydrogen is trapped by a Mo atom so as to reduce thelattice distortion around the Mo atom, and is located at asite displaced from a T site by about 0.6 Å (Ttr site) towards
the Mo atom, indicating a strong attractive interaction
between hydrogen and Mo atoms.3) With increasing CMo,the lattice distortion is reduced owing to interference
between strain fields around individual Mo atoms, and mostof the H atoms occupy T sites as in Nb. For CMo>39 at. %,the lattice distortion gradually increases again withincreasing CMo, because of an increase in the number of Condensed from the article in J. Phys. Soc. Jpn. 83 064602 (2014).
RIKEN Nishina Center䠆2
School of Science and Engineering, Waseda University.
䠆3
Department of Physics II, Tokyo University of Science.
䠆4
Institute for Materials Research, Tohoku University.
䠆1- 227 -undersized Mo atoms in a unit cell, but not so strongly asthat at low CMo, i.e., up to an intermediate level. In this case,hydrogen is distributed over T and d-T sites, where d-T sites
are displaced from T sites to the nearest neighbour
octahedral (O) sites by about 0.25 Å. Hydrogen
preferentially occupies T sites in the undistorted or very
weakly distorted region, but, as the concentration ofavailable T sites is limited, excess H atoms occupy d-T sites
in the region distorted at the intermediate level.2)
In the Nb-Ta alloys, at low CTa, hydrogen is distributed
over T and d-T sites as in the case of the Nb-Mo alloys with
CMo>39 at. %. At low CTa, the lattice distortion wasobserved to be at the intermediate level from themeasurement of X-ray reflection line width. In the 50 at. %Ta alloys, where lattice is less distorted, hydrogen is locatedat T sites. Therefore, there exists no attractive interaction
between hydrogen and an oversized Ta atom.4)
According to the above-stated interpretation, it is expected
that, in the Ta-rich Ta-Nb alloys, hydrogen is distributed
over T and d-T sites, because a size difference between Nb
and Ta atoms is small, although a Nb atom is an undersized
solute. In this study, therefore, the lattice location ofhydrogen in the Ta-rich Ta-Nb alloy with 3 at. % Nb wasinvestigated. This alloy is symmetrical in the soluteconcentration to the hitherto studied Nb-rich Nb-Ta alloys
containing 2 or 5 at. % Ta atoms. The following results
were obtained. Hydrogen atoms occupy T and d-T sites,
which are displaced by 0.25±0.3 Å from T-sites. Hydrogen
preferentially occupies T sites and excess H atoms enter d-T
sites. This is compatible with the above-stated expectation,and is different from the case in Nb alloyed with 3 at. %undersized Mo atoms. There exists no strong attractive
interaction between hydrogen and undersized Nb atoms inTa. Considering also the results obtained for the Nb-Mo
alloys, the Nb-50 at. % Ta and Nb-(2±5) at. % Ta alloys, it
is concluded that, in the Nb-Mo and Nb-Ta alloys, the site
occupancy of hydrogen and the interaction betweenhydrogen and solute atoms can be well explained on thebasis of the lattice distortion induced by alloying.
References
1) E. Yagi, T. Kobayashi, S. Nakamura, Y. Fukai, and K.
Watanabe: J. Phys. Soc. Jpn. 52, 3441 (1983).
2) E. Yagi, et al.: J. Phys. Soc. Jpn. 79, 044602 (2010).
3) E. Yagi, S. Nakamura, F. Kano, T. Kobayashi, K. Watanabe, Y.
Fukai, and T. Matsumoto: Phys. Rev. B 39, 57 (1989).
4) C. Sugi, E. Yagi, Y. Okada, S. Koike, T. Sugawara, T. Shishido
and T. Ogiwara: J. Phys. Soc. Jpn. 82, 074601 (2013).