Impact of electronic structure calculations on study of diffusion in metals

Developments in the methods of electronic structure calculations combined with the impressive advances in computer performance have made it possible to perform ab initio calculations of a variety of structural properties of materials efficiently and at a predictable level, as illustrated here in the case of defect parameters in transition metals. The values of the vacancy formation and migration enthalpies calculated in the framework of the density functional theory in the local density approximation agree with experimental data to typically within 10%. Reliable self-diffusion coefficients for the vacancy mechanism can be deduced. This approach sheds new light on the so called self-diffusion anomaly in bcc metals. It outlines in particular the role of structural relaxations around the vacancy and it reveals that electronic excitations may make a contribution to the formation and migration entropies comparable with the vibrational contribution. The link between the electronic structure and vacancy properties in bcc transition metals is analysed using a simple tight binding d band model: the position of the Fermi level with respect to the characteristic dip in the density of states is shown to govern the sharp variations along a transition metal series of the vacancy parameters.