Vacancy-Formation Thermodynamics in Aluminium and Nickel: A Computational Study

Although anharmonic effects on the properties of point defects have been considered in the past, only recent progress in simulation techniques have come to show their importance. Using molecular dynamics and nonequilibrium free-energy techniques we investigate such effects on the formation free energies of vacancies and divacancies in aluminium and nickel represented by many-body interatomic potentials. Furthermore, we discuss the applicability of the harmonic approximation to compute defect formation entropies for the used potentials. It is found that the harmonic approximation may fail to give a correct description of defect-formation entropies, even at low temperatures. Using the results of the calculations including all anharmonic effects, we analyze the stability of point defects and interpret the results in terms of the curvature that some fee metals exhibit in the high temperature range of the equilibrium vacancy concentration Arrhenius plots.