AB-Initio Based Calculations of Vacancy Formation and Clustering Energies Including Lattice Relaxation in Fe3Al

Vacancy formation and clustering significantly affect structural properties of transition-metal aluminides. Ab-initio quantum mechanical total-energy calculations using a full-potential linear combination of muffin-tin orbitals (LMTO) technique provide a convenient method of studying relevant characteristics such as changes in density of states, and charge redistribution around defects. Augmented with Hellmann-Feymann forces, LMTO allows calculations of relaxation geometries and relaxation energies. We have performed such calculations for vacancies and antisite substitutional point defects in Fe3Al with DO3 crystallographic structure. There are two limiting factors complicating calculations of defect formation energies directly from ab-inito calculations. The first is that a single defect, due to the lattice periodicity necessitated by the use of ab-inito total energy techniques, cannot be considered as an isolated defect, even in the maximum computable simulation cell. Unlike previous calculations [ I ], which did not find a dependency on the size of the simulation cell, our calculations have shown a significant difference in results for 32- and 16- atom cells. This difference provides information about vacancy clustering since it can be explained by a relatively small attractive interaction energy ~0.2 eV between two vacancies located in adjacent simulation cells and separated by the lattice constant distance (5.52Å) [2]. By comparing the internal energies for two configurations of 30 atom cells (32 atom - 2 vacancies) we were able to estimate that the attractive interaction between two vacancies could reach 1.2 eV. The second complication is the fact that chemical potentials of elements cannot be directly extracted from the total energy calculations for the compound. To deal with this problem, we considered two possible approximations and compared results, which were found to be quite similar for iron vacancies.