Finite temperature vacancy formation thermodynamics: Local harmonic and quasiharmonic studies

The vacancy formation thermodynamics in six FCC metals Ag, Au, Cu, Ni, Pd and Pt are determined from atomistic simulations as a function of temperature. The investigation is performed using the embedded atom method interatomic potentials and the finite temperature properties are determined within the local harmonic and the quasiharmonic frameworks. The temperature dependence of the vacancy formation free energy, entropy, enthalpy and vacancy formation volume are determined. The authors find that the temperature dependence of the vacancy formation energy can make a significant contribution to the vacancy concentration at high temperatures. An additional goal of the study is to evaluate the accuracy of the local harmonic method under circumstances in which the excess entropy associated with the formation of a defect is very small. The data demonstrate that while the errors associated with determining the vacancy formation entropy in the local harmonic model are large, a simple extension to the local harmonic method yields thermodynamic properties comparable to that obtained in the quasiharmonic model, but with much higher computational efficiency.