PROPERTIES OF MONOVACANCIES AND SELF-INTERSTITIALS IN BCC NA - AN AB-INITIO PSEUDOPOTENTIAL STUDY

By means of ab initio pseudopotential theory and the local-density approximation various properties of monovacancies and self-interstitials in bcc Na, viz. the formation energies and formation volumes and the electric field gradients in the neighborhood of the defects, are calculated. As in the case of Li treated earlier, comparison with experimental results leads to the following conclusions. (i) An interstitialcy mechanism of self-diffusion may be excluded. (ii) The vacancy formation enthalpy is so close to the activation enthalpy of self-diffusion that either the vacancy migration enthalpy must be extraordinarily small (this raises the question of the validity of the transition state theory) or the low-temperature self-diffusion in Na is due to a direct exchange of neighboring atoms or to a ring mechanism involving three atoms. A nonvacancy mechanism is supported by the fact that the calculated monovacancy formation volume at 0.51 atomic volumes is substantially larger than the experimentally determined activation volume of the low-temperature mechanism of self-diffusion.