Abstract
An atomistic model has been used to calculate the dipole tensor of vacancies and impurity-vacancy (IV) pairs in ionic crystals. The program PDINT, developed for f.c.c. ionic crystals and based on a generalized Mott-Littleton procedure, was used. Two methods of evaluating dipole tensors of defects were investigated, the direct-moment and the strain-derivative methods. A comparison of the results of the two approaches showed that the strain-derivative method is more reliable and convenient for ionic crystals, particularly for extended anisotropic defects, such as IV-pairs. The dipole tensors of Ca2+ IV nearest-neighbour (n.n.) and next-nearest neighbour (n.n.n.) pairs in NaCl were calculated. The results show that the n.n.n. pair has tetragonal symmetry, whereas the n.n. pair has orthorhombic symmetry. Converted to the strain tensor form, the tetragonality of the n.n.n. pair ranges from 0.16 to 0.22, depending on the potential. The symmetry of the n.n. pair is close to tetragonal, yielding an approximate tetragonality of 006.
Changes in the elastic constants of an MgO crystal resulting from the addition of cation and anion vacancies were calculated using the strain-derivative method. The results were used to improve a previous linear, elastic point-defect/dislocation interaction model, and the predictions of the elastic model were compared with previously determined atomistic values. The comparison shows that non-linear interaction effects are still significant, even at distances along the slip plane of four Burgers vectors away from the centre of the dislocation.
