Ab-initio study of the transition pathways for single and double interstitial solute (H, N, O, H-H, N-N, and O-O) within bcc refractory metals (Mo and Nb)

Transition pathways of single (Hydrogen (H), Nitrogen (N), and Oxygen (O)) and double (H-H, N-N and O-O) interstitial solutes within bcc refractory metals (molybdenum (Mo) and niobium (Nb)) were investigated. This work is crucial for understanding how atmospheric gases, rich in H, O, and N, interact with metals. Ab-initio calculations for equilibrium and structural parameters, dissolution energetics, charge transfers, minimum energy path, and diffusion coefficients were performed. Single solutes exhibited preferential occupancy sites, with H favoring tetrahedral sites (t-sites), N preferring octahedral sites (o-sites), and O showing material-dependent behavior. The energy barriers for single solute diffusion ranged from 0.10 to 1.34 eV, aligning with experimental findings. Double interstitial solutes significantly reduced activation energies (E ), leading to faster diffusion for all configurations except for MoO. This effect is due to the second solute’s influence on repulsive/attractive forces and local lattice relaxations, altering preferred diffusion pathways.