The influence of irradiation induced vacancies on the mobility of helium in boron carbide

Abstract Boron carbide, used as a neutron absorber, undergoes nuclear reactions producing relevant quantities of He. The understanding of He kinetics at the atomic scale in the material is still in its infancy, in spite of decades of experimental work devoted to the characterization of He containing, irradiated, boron carbide samples. The interplay of He itself with intrinsic defects created by irradiation on kinetics is still almost completely unknown. In this paper we present an exhaustive study of vacancies and substitutional helium impurities in boron carbide using Density Functional Theory. Analyzing the stability and mobility of these defects allows us to consider diffusion mechanisms other than the known interstitial mechanisms. We find that vacancies trap He interstitials, raising the activation energy of 2D diffusion to approximately 2 eV. The trapping mechanism is different according to the charge state of the vacancy: in p -type conditions, when vacancies are neutral or positive, He diffuses via a dissociative mechanism and is trapped in a substitutional position; in n -type conditions, negative vacancies trap He atoms traveling in an adjacent { 111 } plane by a charge transfer driven distorsion. No favorable vacancy assisted diffusion mechanism was identified for substitutional He atoms, except the dissociative one previously mentioned. Other possible vacancy diffusion mechanisms, which we also analyzed, are hindered by the high activation energy of vacancy self-diffusion.