Energetics and structural relaxation of constitutional defects in CoAl and CoTi from first principles

The energetics and structural relaxations of constitutional defects in CoAl and CoTi are investigated using first-principles electronic structure calculations. In order to estimate the stability of the constitutional defects, the compositional dependence curves of the formation energy are obtained from the calculations employing supercells of various sizes. The difference in the bonding character induces the difference in the energetics and structural relaxations of the defects between CoAl and CoTi: A covalent bonding character is more pronounced in CoAl than in CoTi and CoTi has a larger ionic bonding character than CoAl. The Co vacancy is energetically more favorable than the Al antisite in Al-rich CoAl, whereas the opposite trend is observed in Ti-rich CoTi. The Ti-d orbital at the antisite and the excess electrons play an important role for the stability of the Ti antisite atom. The relaxation around the constitutional defects in CoAl and CoTi cannot be explained by only the atomic size difference. The change in the bonding charge density as a result of the charge redistribution around the defects plays a key role in the structural relaxation. This situation originates from the mixture of the ionic and covalent bonding character in intermetallic compounds. The ionic bonding character leads to the effective charge around the defects and the covalent bonding is affected by the charge redistribution screening the effective charge. The ionic bonding character in CoTi brings about the outward relaxations of the neighboring atoms around the Co antisite atom and the Ti vacancy in CoTi.