24PO66-P
Study on hydrogen-vacancy complexes in BCC metals from first-principles
culculation
第一原理計算によるBCC金属中の空孔水素複合体に関する研究
Keisuke EGUCHI1, Kazuhito OHSAWA2, Masatoshi YAGI2
Kyushu Univ.1, RIAM Kyushu Univ.2
江口	  佳佑1, 大澤	  一人2, 矢木	  雅敏2九州大学総合理工学府1, 九州大学応用力学研究所2
Kyushu Univ.
6-1, Kasugakouen, Kasuga, Fukuoka 816-8580, Japan
九州大学大学院 総合理工学府 〒816-8580	  福岡県春日市春日公園6-1
It has been reported that a maximum of six hydrogen atoms are trapped in a BCC metalmonovacancy, but a maximum of twelve hydrogen atoms can be accommodated in a tungsten
and molybdenum monovacancy. It is found that the stable positions of H atoms shift toward
tetrahedral interstitial sites as the number of hydrogen atom increases.

1. Introduction
Plasma facing materials (PFMs) are
exposed to high heat and high energy
particles for a long time period in plasma
confinement devices, such a fusion reactor.
As a result, many lattice defects are
nucleated in the PFMs by collisions of theplasma particles. In addition to the direct
damage, the accumulation and retention ofhydrogen (H) isotopes in metals induceembrittlement of the PFMs. We simulate
stable H sites trapped in molybdenum (Mo)and tungsten (W) monovacancy in terms offirst-principles calculations in order toinvestigate the properties of W-H and Mo-Hsystems. For comparison Fe-Hsystem is also
examined.
In the case of BCC lattice, such as Mo andW, an interstitial atom is located attetrahedral interstitial sites, as shown inFIG. 1 (a). On the other hand, a single H
atom trapped a Mo and W monovacancy islocated close to an o-site on the inner surfaceof the monovacancy as shown in FIG. 1 (b).
Similarly, multiple H atoms (n≦6) trapped ina monovacancy have been also assume to belocated close to the o-sites. However, we areskeptical about the established assumption.
In order to determine valid stableconfigurations of multiple H atoms in amonovacancy, a skillful calculation method
was developed in the present work.

FIG. 1. (a) Schematic view of o-site ( ) and t-site (). (b) Schematic view of a single H trapped in avacancy in BCC lattice.

2. Calculation method
First-principles calculations based on thedensity functional theory are performed
using the Vienna ab initio simulation
package (VASP) with the generalized
gradient approximation and projectedaugmentedwavepotentials.
Periodic
boundary condition is imposed to a BCC
supercell composed of 54 lattice points.
Plane-wave energy cutoff is 350 eV and 5*5*5
Brillouin zone sampling is used. The lattice
relaxation is iterated until each resultant
force applied on each atom becomes lower
than 0.003 eV/Å.
Total binding energy is defined as
Etotal = E[M53V] − E[M53VHn] ＋ n(E[M54HT] −E[M54]), (1)where the function E is the cohesive energyof the supercell. M53V is supercell composedof 53 metal atoms and a vacancy; M53VHn isthat composed of 53 metal atoms, a vacancy,and n H; M54HT is that composed of 54 metal
atoms and one H embedded at a T-site; M54 isperfect lattice composed of 54 metal atoms,respectively.
We investigated the more stable sites forthe multiple H atoms using random initial
distribution of n H atoms (n ≦ 6) in amonovacancy, as shown FIG. 2. In the case n≧7, we located random initial distribution
close to t-sites.

atoms is trapped in a W and Mo monovacancy,
as shown in FIG. 4.	  While, an Fe
monovacancy can accommodate only six Hatoms.

FIG. 2. Schematic view of initial distribution of n
H atoms in a metal vacancy. Random placementof an H atom is uniformly distributed in each graysquare. Center of square is close to the o-site.

3. Results
It is found that the stable sites of multiple
H atoms trapped in Mo, W and Fe
monovacancy are shifted toward tetrahedral
interstitial sites as the number of H atomsincreases. H configurations in a Mo
monovacancy are shown in FIG.3 as anexample.

FIG. 4. Total binding energy as function of H
atom number trapped in W, Mo and Fe vacancy.

4. Discussion
The H solubility for Mo and W is lower
than that for Fe. However, too many H atoms
are trapped in their monovacancy comparedwith other BCC metals. We suppose thereason is that the H atoms are strongry
segregated to the vacancy-type lattice defects
to reduce the high solution energy.
Zero point energy (ZPE) corrections for
Metal-H systems are not necessarilynegligible compared with their binding
energies. However, ZPE do not play anessential role to determine the important andfundamental properties of the systems, e.g.
ground-state configurations of H atoms in amonovacancy etc.
References
1Y.–L.

FIG. 3. Stable points of H atoms projected onto
the {001} plane. Circle, triangle, square, anddiamond systems correspond to the cases where
one, four, six, and twelve H atoms are trapped ina monovacancy in Mo lattice, respectively. O-site
is located at the center of figure.

It is understood that a maximum of twelve H

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Liu, and G.–N. Luo, Phys. Rev. B 79, 172103(2009)
2M. Yamakami, M.S. thesis, Kyushu University,
2009; M. Yamakami, M. Yagi, and K. Ohsawa,
Rep. Res. Inst. Appl. Mech. (Kyushu Univ.) 137,81-99 (2009)
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