Vacancy self-diffusion parameters in tungsten: Finite electron-temperature LDA calculations

An ab initiopseudopotential study of the monovacancy properties in bcc tungsten is presented. The formation and migration enthalpies are calculated for relaxed configurations using supercells containing up to 54 atomic sites, both in the electronic ground state and at finite electron temperature. The electronic contribution to the formation entropy — usually neglected in point defect calculations — is shown to be positive and equal to 1.74kB 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 calculated values of the migration and formation enthalpies are found to be in excellent agreement with experiments at low temperatures, and their significant quadratic temperature dependence — due to electronic excitations — is shown to explain part of the experimentally observed temperature dependence of the migration enthalpy and self-diffusion activation energy. The tracer self-diffusion coefficient is calculated within the rate theory: the Arrhenius slope is in excellent agreement with experiments, and so are the absolute values provided that the electronic entropies are taken into account.