Electron localization on dislocations in metals: Real-space first-principles calculations

The electronic structure of different types of dislocations in $B2$ intermetallic NiAl and bcc transition metals was investigated using the first-principles real space tight-binding linear-muffin-tin-orbital recursion method. An unusual localization of electronic states inside the valence band is observed in the cores of $〈100〉${010} and $〈100〉${011} edge dislocations but not in $〈111〉${011} edge dislocations. The nature of these localized electronic states and mechanisms for their appearance are analyzed. We show that conditions of electron localization are (i) a decrease of the number of nearest neighbors (``broken bonds'') around the central atom of the dislocation core, (ii) a specific local symmetry of the atomic arrangement in the region of the dislocation core, and (iii) the contribution of d states to the formation of these interatomic bonds. We illustrate our conclusions by demonstrating that electron localization also occurs in $〈100〉${010} edge dislocations in bcc metals W, Fe, and Ni. In contrast to semiconductors, the electron localization in metals takes place on bonding orbitals, and will therefore have a significant impact on dislocation energetics and chemical bonding. We suggest that localized electronic states can give a significant contribution to impurity-dislocation interactions in NiAl and other $B2$ intermetallics such as CoTi, CoHf, and CoZr.