First-principles study of the thermodynamics of hydrogen-vacancy interaction in fcc iron

The interaction of vacancies and hydrogen in an fcc iron bulk crystal was studied combining thermodynamic concepts with ab initio calculations and considering various magnetic structures. We show that up to six H atoms can be trapped by a monovacancy. All of the studied point defects (single vacancy, H in interstitial positions, and H-vacancy complexes) cause an anisotropic elastic field in antiferromagnetic fcc iron and significantly change the local and total magnetization of the system. The proposed thermodynamical model allows the determination of the equilibrium vacancy concentration and the concentration of dissolved hydrogen for a given temperature and H chemical potential in the reservoir. For H-rich conditions a dramatic increase in the vacancy concentration in the crystal is found.