First-principles study of the interaction of nitrogen with transition metal solutes in tungsten

The choice of tungsten (W) as the plasma-facing material requires seeding impurities into the edge plasma for radiative cooling, and gaseous nitrogen (N) is one of the most likely impurities to be used for this purpose. In this work, first-principles calculations related to the interactions between N and transition metal (TM) solutes were performed. The interactions between the TM solutes and N are predominantly attractive and very localized. Specifically, the 3d solutes exhibit a stronger attraction to N than the 4d and 5d solutes. Consequently, most TM solutes can alter the N distribution, promoting the N aggregation within their neighboring shells, and impeding the N diffusion. Moreover, this effect is more conspicuous in the 3d solutes related to the 4d and 5d solutes. In the Sol-Vac-N complexes, the vacancy assumes a predominant role. Most TM solutes diminish the capability of vacancies to capture N. The exception arises in Ti, V, Zr, Nb, Hf, and Ta, where these solutes enhance the vacancies' ability to capture N. Further investigation revealed that, for Ti, V, Zr, Hf, Ta, and Os, the capacity of Sol-N pairs to adsorb the additional N and solute atoms is significant. As a result, it can be speculated that the Sol-N pairs for these solutes might continue to adsorb additional N and solute atoms, eventually evolving into nitrides. However, for the primary transmutation product Re, the capacity of Sol-N pairs to adsorb additional Re atoms is negligible. Therefore, it is inferred that the formation of Re nitride is rather unlikely during W's service life as plasma-facing materials.