Ab initio study on divacancy binding energies in aluminum and magnesium

The divacancy binding energies in fcc Al and hcp Mg have been examined by ab initio calculations based on the density-functional theory. Dense k-point meshes and large supercells are used enough to obtained converged values. For Al, the first-nearest-neighbor (1NN) divacancy is indeed unstable with a negative binding energy similar to the recent calculations by Carling [Phys. Rev. Lett. 85, 3862 (2000)]. However, the second-nearest-neighbor (2NN) divacancy is stable with a positive binding energy larger than their value. For Mg, both the 1NN and 2NN divacancies corresponding to the 1NN divacancy in fcc structures are stable with positive binding energies in accordance with the conventional view. The difference in the stability of divacancies in Al and Mg has been analyzed clearly through the calculations of divacancies in hcp Al and fcc Mg. The tendency to form local directional bonds with covalency at defects is the electronic origin of the peculiar nature of divacancies in Al.