The configuration of multiple hydrogen atoms trapped in a tungsten monovacancy is investigated using first-principles calculations. Unlike previous computational studies, which have reported that hydrogen in bcc metal monovacancies occupies octahedral interstitial sites, it is found that the stable sites shift toward tetrahedral interstitial sites as the number of hydrogen atoms increases. As a result, a maximum of twelve hydrogen atoms can become trapped in a tungsten monovacancy.

We present a first-principles study of stable configurations of single and multiple H atoms in a monovacancy in bcc transition metals and binding energies of the H atoms to the monovacancy. Typical bcc transition metals are group-V elements (V, Nb, and Ta), group-VI elements (Cr, Mo, and W), and Fe. The most stable site for an interstitial H atom in the intrinsic bcc transition metals is a tetrahedral interstitial site (T site). On the other hand, a single or a few H atoms trapped in a monovacancy in bcc metals occupy close to octahedral interstitial sites (O sites) next to the monovacancy. However, stable configurations of four and more-than-four H atoms in the monovacancy are various and different depending on the host metals. Stable sites for H atoms are usually shifted toward the T site or diagonal interstitial site (D site) as the number of H atoms increases in the monovacancy. As a result, a maximum of six H atoms can be accommodated in a monovacancy in V, Nb, Ta, Cr, and Fe, which is in good agreement with previous computational studies, while 10 and 12 H atoms can be accommodated in a Mo and W monovacancy, respectively.

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