The effect of antisite defects on the mechanical and dynamic stability of NbAl.

The A15-type conventional superconductor NbAl alloys has been considered as an ideal candidate for next generation high field magnets due to its higher superconducting properties and less sensitivity to stain than that of industrialized NbSn superconductor. First-principles methods are employed to study the potential point defects, vacancy and antisite defects in deviating stoichiometric NbAl alloys and their effect on structure and mechanical properties. Our results show that antisite defects are easier to be produced than vacancy defects, and NbAlantisite defects can keep the tetragonal structure of NbAl. Furthermore, the influence of antisite defects on dynamic stability of NbAl is investigated together with NbAldefects. With the increase of Nb antisite defect content and the formation of orderly arrangement, we found the phonon spectrum yields no more soft phonon modes, which is in contradiction with the dynamical instability of stoichiometric NbAl with no defects. Our calculations indicate Nb antisite defects play a crucial role on the dynamic stability of NbAl compounds.