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

RIKEN Accel. Prog. Rep. 49 (2016)

Site change of hydrogenhydrogen owing
owing toto lattice
lattice distortion
distortion inin Nb
Nb alloys ††C. Sugi,*1, *2 E. Yagi,*1, *2 Y. Okada,*2 S. Koike,*3 T. Sugawara,*4 T. Shishido,*4 and K. Ogiwara*1
An interaction of hydrogen with solute atoms is one of
the fundamental problems on hydrogen in metals, because
various hydrogen-related properties are strongly affected byalloying. In order to understand the interaction, theknowledge of the atomistic state of hydrogen in alloys ishighly required. However, such information has been
extremely limited, because of experimental difficulties.
Therefore, the channelling method utilizing a nuclear
reaction of 1H(11B,a)aa with a 11B+ beam of about 2 MeV
had been newly developed.1) Hydrogen can be detected bymeasuring emitted a particles. This method has been
demonstrated to be very useful to locate hydrogen dissolved
in metals.1,2) The lattice location of hydrogen has hitherto
been investigated systematically in detail in Nb alloyed with
undersized Mo atoms up to 60 at. % by the channelling
method at room temperature with a tandem accelerator.
Their atomic radii are 1.43 Å for Nb and 1.36 Å for Mo
atoms. This alloy system forms a solid solution over theentire Mo concentration (CMo) range, maintaining a bcc
crystal structure, although the lattice parameter changes.
It has been demonstrated that the lattice location changes
very sensitively with CMo and, with the help of
measurement of width of X-ray reflection lines (Fig. 1), thatsuch change can be interpreted in terms of lattice distortion
induced by alloying.3) Broadening of reflection lines serves
as a measure of lattice distortion averaged over the wholespecimen. At low CMo, lattice is strongly distorted aroundMo atoms. Hydrogen is trapped by a Mo atom to be locatedat a Ttr site, which is displaced from an original tetrahedral(T) site by about 0.6 Å towards the Mo atom, so as toreduce the distortion around Mo atom. There exists a strong
attractive interaction between hydrogen and Mo atoms.4)
With increasing CMo, the lattice distortion is reduced owing
to interference between strain fields around individual Moatoms, and most of the H atoms occupy T sites as in Nb. For
CMo higher than 39 at. %, the lattice distortion graduallyincreases again with increasing CMo because of the increasein the number of undersized Mo atoms in a unit cell, but not
so strongly as that at low CMo, i.e., up to an intermediatelevel (the range II in Fig. 1). In this case, H atoms aredistributed over T and d-T sites, which are displaced from T
sites to their nearest neighbour octahedral (O) sites by about
0.25 Å. The T-site occupancy is energetically morefavourable than the d-T-site occupancy. Hydrogen
preferentially occupies T sites in undistorted or very weakly
distorted region, but, as the concentration of available T†

Condensed from the article in J. Phys. Soc. Jpn. 82 074601 (2013).

RIKEN Nishina Center
School of Science and Engineering, Waseda University.
＊3
Department of Physics II, Tokyo University of Science.
＊4
Institute for Materials Research, Tohoku University.
＊1＊2

sites is limited, excess H atoms occupy d-T sites in theregion distorted at the intermediate level.3) Therefore, it isexpected that H atoms are located at T sites in the alloys
whose lattice is very weakly distorted.
Then, in the present study, the site occupancy ofhydrogen in Nb alloyed with 50 at. % of oversized Ta atoms
with a very closer atomic radius to Nb atoms, 1.44 Å,
Nb0.50Ta0.50H0.021 and Nb0.50Ta0.50H0.053, was investigated by
the channelling method at room temperature. It wasobserved that H atoms are located at T sites, irrespective ofhydrogen concentration (CH), in contrast with the results onNb alloyed with similar concentration of undersized Moatoms, Nb0.52Mo0.48 (CH=0.0097–0.05). In approximately
the same CH range as that in the Nb0.50Ta0.50 alloys, in theNb0.52Mo0.48 alloys, at low CH, most of the H atoms arelocated at T sites and excess H atoms are at d-T sites, whileat high CH, most of them are at d-T sites.3)
Widths of X-ray diffraction lines in the Nb0.50Ta0.50 alloys
are much smaller than those in the Nb0.52Mo0.48 alloys andare very close to those in pure Nb (Fig. 1), indicating muchsmaller lattice distortion in the Nb0.50Ta0.50 alloys than in theNb0.52Mo0.48 alloys, owing to the smaller size difference
between Nb and Ta atoms. The present result on site
occupancy of hydrogen in the Nb0.50Ta0.50 alloys is in good
agreement with the expectation. Therefore, considering also
the hitherto obtained results for Nb alloyed with 2–5 at. %Ta atoms, it is concluded that the site occupancy ofhydrogen in Nb-Mo and Nb-Ta alloys can be well explainedon the basis of the lattice distortion induced by alloying.

Fig. 1. The half-widths of the {321} X-ray reflection linesof the non-H-doped Nb-Mo (●) and Nb-Ta (▲) alloys. Opensymbols, Nb-Mo (○) and Nb-Ta (▽), represent those byMatsumoto et al.5)
References

1) E. Yagi. T. Kobayashi, S. Nakamura, Y. Fukai, and K.
Watanabe: J. Phys. Soc. Jpn. 52, 84 (1983).
2) E. Yagi, T. Kobayashi, S. Nakamura, Y. Fukai, and K.
Watanabe: Phys. Rev. B 31, 1640 (1985).
3) E. Yagi et al.: J. Phys. Soc. Jpn. 79, 044602 (2010).
4) E. Yagi, S. Nakamura, F. Kano, T. Kobayashi, K. Watanabe, Y.
Fukai, and T. Matsumoto: Phys. Rev. B 39, 57 (1989).
5) T. Matsumoto et al.: J. Phys. Chem. Solids 36, 215 (1975).

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