Self-interstitial atom defects in bcc transition metals: Group-specific trends

We present an investigation of systematic trends for the self-interstitial atom (SIA) defect behavior in body-centered cubic (bcc) transition metals using density-functional calculations. In all the nonmagnetic bcc metals the most stable SIA defect configuration has the $⟨111⟩$ symmetry. Metals in group 5B of the periodic table (V, Nb, Ta) have significantly different energies of formation of the $⟨111⟩$ and $⟨110⟩$ SIA configurations, while for the group 6B metals (Cr, Mo, W) the two configurations are linked by a soft bending mode. The relative energies of SIA defects in the nonmagnetic bcc metals are fundamentally different from those in ferromagnetic bcc $\ensuremath{\alpha}$-Fe. The systematic trend exhibited by the SIA defect structures in groups 5B and 6B transition metals correlates with the observed thermally activated mobility of SIA defects.