Ab initio study of self-interstitials in hcp-Zr

The formation energies of self-interstitials in hcp-zirconium have been obtained from first principles electronic structure calculations in the framework of the density functional theory in the local density approximation using a planewave pseudopotential approach with supercells containing 37 and 97 atoms. For the largest cell, five configurations are found to be nearly degenerate in energy. The most stable ones are the octahedral (O), split 〈0001〉 dumbbell (S), and basal octahedral (BO) configurations, with formation energies E_fⁱ=2.8 eV, and the basal split (BS) with E_fⁱ=2.9 eV. The crowdion (C) configuration is slightly less stable with E_fⁱ=3.1 eV, while the basal crowdion (BC) is unstable and decays to BO. The effect of the exchange correlation functional is investigated by repeating the same calculations for 37 atom supercells within the generalized gradient approximation (GGA): the formation energies increase by almost 0.2 eV for the basal configurations and by more than 0.3 eV for the non-basal ones. From the analysis of the present results, the most stable configuration in hcp-Zr at low temperature is predicted to be BO, but BS and to a lesser extend O and S cannot be excluded.