Vibrational contributions to the stability of point defects in bcc iron: A first-principles study

The purpose of this study is to investigate the modes of vibration of the self-interstitial atoms and the vacancy in bcc iron and to estimate how the vibrational properties can affect the stability of these defects. The phonon density of states of the vacancy and the self-interstitials have been calculated within the quasi harmonic approximation using density functional theory calculations. It was observed that self-interstitial atoms have several localized high frequency modes of vibration related to the stretching of the dumbbell bond, but also soft modes favoring their migration. From the phonon density of states, the vibrational contributions to the free energy have been estimated for finite temperatures. Results are compared to previous work performed by others using empirical potentials. We found a rather large formation entropy for the vacancy, S-V(f) = 4.08 k(B). Our results show that the vibrational entropy can have a significant influence on the formation of the point defects even at moderate temperature. Possible consequences on the mobility of these defects are also discussed. (C) 2009 Elsevier B.V. All rights reserved.