Vacancies and interstitials, with the associated forma­tion energy driving their concentration and migra­tion energy driving their displacement in the solid; the sum of these two energies is the activation energy for diffusion at equilibrium. For such simple defects, it is possible to go beyond the 0 K energies and to access the free energies of formation and migrations by calculating the vibrational spectra in the presence of the defect in the stable position and at the saddle point (see Section 1.08.4.2.3).

1.08.3.2.1 Hetero-defects

In the nuclear context, such defects can be fission products in a fuel material, actinide atoms in a waste material, helium gases in structural materials, and so on; ab initio gives access to the solution energy of these impurities, which allows one to determine their most favored positions in the crystal: interstitial position, substitution for host atoms, and so on. The kinetic energies of migration of interstitial impurities are accessible as well as the kinetic barrier for the extraction of an impurity from a vacancy site.