VACANCY SELF-DIFFUSION PARAMETERS IN TUNGSTEN - FINITE ELECTRON-TEMPERATURE LDA CALCULATIONS

An ab initio pseudopotential study of the monovacancy properties in bc c tungsten is presented. The formation and migration enthalpies are ca lculated for relaxed configurations using supercells containing up to 54 atomic sites, both in the electronic ground state and at finite ele ctron temperature. The electronic contribution to the formation entrop y - usually neglected in point defect calculations - is shown to be po sitive and equal to 1.74k(B) at melting temperature. This large value is related to peaks in the electronic density of states just below the Fermi level due to states localized around the vacancy. The calculate d values of the migration and formation enthalpies are found to be in excellent agreement with experiments at low temperatures, and their si gnificant quadratic temperature dependence - due to electronic excitat ions - is shown to explain part of the experimentally observed tempera ture dependence of the migration enthalpy and self-diffusion activatio n energy. The tracer self-diffusion coefficient is calculated within t he rate theory: the Arrhenius slope is in excellent agreement with exp eriments, and so are the absolute values provided that the electronic entropies are taken into account.