Configuration and binding energy of multiple hydrogen atoms trapped in monovacancy in bcc transition metals

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.