Thermodynamics of hydrogen-induced superabundant vacancy in tungsten

We investigate superabundant vacancy formation induced by hydrogen in tungsten in terms of an equilibrium thermodynamic model to estimate hydrogen isotope retention in plasma facing materials. Vacancy-hydrogen cluster concentrations in the bulk tungsten are calculated as a function of the H concentration at finite temperature. A monovacancy in usual bcc transition metals is capable of accommodating six H atoms, while a maximum of 12 H atoms can be accommodated in a tungsten monovacancy, according to first-principle calculations. The present results provide thermodynamic profiles of vacancy-hydrogen clusters trapping more than six H atoms for the first time. In present work, configurational transitions of H atoms trapped in the monovacancy and activation energies for them are investigated by examining the transition paths in order to calculate configurational entropy. Vacancy-hydrogen clusters trapping more than six H atoms exist in thermodynamic equilibrium. However, the major vacancy-hydrogen clusters are composed of six H atoms in a wide range of temperature and H concentration.