The energetics and structural relaxation of constitutional defects were investigated by using first-principles electronic structure calculations. In order to estimate the stability of the constitutional defects, the compositional dependence curves of the formation energy were deduced from the calculations performed by using super-cells of various sizes. The difference in the bonding nature led to differences, between CoAl and CoTi, in the energy and structural relaxation of defects. Covalent bonding was more pronounced in CoAl, than in CoTi, and CoTi had a more ionic bonding nature than CoAl. The Co vacancy was energetically more favorable than the Al antisite in Al-rich CoAl, whereas the opposite trend was observed in Ti-rich CoTi. The Ti-d orbital at the antisite, and the excess electrons, played an important role in the stability of the Ti antisite atom. The relaxation around the defects in CoAl and CoTi could not be explained only by the atomic size difference. The change in the bonding charge density, as a result of charge redistribution around defects, played a key role in the structural relaxation. This situation arose from the mixture of ionic and covalent bonding natures in intermetallic compounds. The ionic bonding nature led to an effective charge around the defects, and the covalent bonding was affected by the charge redistribution which screened the effective charge. The ionic bonding nature in CoTi caused outward relaxation of the neighboring atoms around the Co antisite atom and the Ti vacancy in CoTi.

Energetics and Structural Relaxation of Constitutional Defects in CoAl and CoTi from First Principles. M.Mizuno, H.Araki, Y.Shirai: Physical Review B, 2003, 68[14], 144103 (14pp)