Investigating behaviors of H in a W single crystal by first-principles: From solubility to interaction with vacancy

We have investigated structure and solubility of H, as well as H-vacancy interaction in tungsten (W) single crystal employing a first-principles method. Single H atom is shown to be energetically favorable sitting at the tetrahedral interstitial site (TIS). The solubility of H is estimated in W according to the Sieverts' law. We found that the solution concentrations are 2.3 x 10(-10) and 1.8 x 10(-7) at the typical temperatures of 600 K and 1000 K, respectively. The calculated results are basically consistent with the experiments. The vacancy can be found to play a key role on the trapping of H in W. There exists a very strong binding between single H and vacancy with the binding energy of 1.18 eV. With the H atoms added, the HnV complexes can be easily formed in the vacancy. A monovacancy is shown to be capable of trapping as many as 7 H atoms. Kinetically, we show that the H jumps into the vacancy from the first nearest neighboring TIS into vacancy with a much reduced barrier of 0.02 eV, which indicates a down-hill drift diffusion of H towards vacancy. The physical mechanism underlying H assisted vacancy formation is originated from that H atoms can stimulate the formation and growth of vacancy or void by binding with vacancy to decrease the effective formation energy of vacancy in W. (C) 2011 Published by Elsevier B. V.