Intrinsic defects and krypton impurity atoms in hcp titanium: A first-principles study

We report on the migration of monovacancies, divacancies, and substitutional krypton atoms in hcp titanium using the nudged elastic band method, in the framework of ab initio density functional theory. The divacancy migration energy barrier is found to be lower than that of a monovacancy. The migration of substitutional krypton in titanium is governed by the vacancy mechanism, if there is an excess of nonequilibrium vacancies after the implantation. However, if thermal vacancies dominate, krypton atom migration is expected to proceed via the dissociation mechanism. We also present ab initio calculations on the formation of clusters of multiple substitutional impurity krypton atoms and the interactions between krypton impurities and vacancies in titanium. We have found that, analogous to multiple interstitial krypton clusters, clusters of substitutional krypton atoms are energetically unstable, but are stabilized by vacancies. However, the efficiency of stabilization by vacancies depends strongly on the spatial distribution of the vacancies within the clusters. This study indicates the possibility of inert gases to nucleate in voids created by vacancies in ion implantation processes.