Interaction between impurity nitrogen and tungsten:a first-principles investigation

We investigate the stability,diffusion,and impurity concentration of nitrogen in intrinsic tungsten single crystal employing a first-principles method,and find that a single nitrogen atom is energetically favourable for sitting at the octahedral interstitial site.A nitrogen atom prefers to diffuse between the two nearest neighboring octahedral interstitial sites with a diffusion barrier of 0.72 eV.The diffusion coefficient is determined as a function of temperature and expressed as D(N)=1.66×10～(-7) exp(0.72/kT).The solubility of nitrogen is estimated in intrinsic tungsten in terms of Sieverts’ law.The concentration of the nitrogen impurity is found to be 4.82×10～(-16) A～3 at a temperature of 600 K and a pressure of 1 Pa.A single nitrogen atom can easily sit in an off-vacancy-centre position close to the octahedral interstitial site.There exists a strong attraction between nitrogen and a vacancy with a large binding energy of 1.40 eV.We believe that these results can provide a good reference for the understanding of the behaviour of nitrogen in intrinsic tungsten.