Theoretical studies of point defects and diffusion in Nb3Sn

The structure and energetics of several simple point defects in A15 Nb3Sn were investigated by means of computer simulations based on a pair-potential model of cohesion. The properties of vacancies on both the Nb and Sn sublattices, as well as those of simple antisite defects, were examined, and estimates were made of the energetics of several types of atom-vacancy exchange ("jump") processes. The results show an unusual structure for the vacancy on the Nb sublattice: the vacancy is "split" between two adjacent sites along the Nb chain with an atom midway between them. We find the Sn vacancy (on the Sn sublattice) to be metastable; this vacancy will "decompose" by an activated process into a more stable configuration consisting of a Nb atom on a Sn site adjacent to a split Nb-sublattice vacancy. We find that the lowest energy grouping of defects compatible with maintaining sublattice sites in the proper three-to-one ratio is the antisite defect pair; the lowest energy grouping which contains vacancies is found to consist of Nb-sublattice vacancies and Nb-on-Sn-sublattice antisite defects in the ratio of four of the former to one of the latter (quintuple defects). Our results also suggest that bulk Sn diffusion is slower than Nb diffusion; this is consistent with the belief that rapid Sn diffusion during Nb3Sn layer growth does not occur by bulk but by grain-boundary diffusion.