Free energies of point defects in sodium from first-principles molecular-dynamics simulations

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Abstract
The formation free energies of the major point defects in sodium are calculated from first-principles (or ab initio$)— molecular-dynamics simulations, which allow for an accurate representation of the electron density around the defect site. Values are obtained for the vacancy, divacancy, and interstitial for a range of temperatures. The vacancy is found to have the lowest free energy of formation at all temperatures except possibly close to the melting point. Vacancies are found to experience an attractive interaction and may stay bound to each other, to form a divacancy, for a significant period of time. Contrary to previous expectations it is found that at temperatures close to the melting point the interstitial may have a significant presence.