The Study Of Formation And Stability Of Irradiation-Induced Defects In Yttria By First Principles

Oxide dispersion-strengthened （ODS） ferritic steels are proposed as thestructural materials for the fourth generation of nuclear reactors due to their excellentmechanical and anti-irradiation properties. These properties are attributed to thepresence of a large number of very small oxide particles dispersed in the ferriticmatrix. The particles are stable under irradiation and they can act as sinks forirradiation-induced defects to resist the effects of irradiation. In order to understandthe anti-irradiation properties of the particles, ab initio calculations have beenperformed to study the formation energies, electronic structures and stability ofvacancies and their clusters in Y₂O₃, which is a constituent of the particles in ODSsteels. In addition, the formation energy and mobility of He as well as the stability ofsmall helium-vacancy clusters are also investigated.The results turn out that, for monovacancies, the formation energies of vacanciesdecrease with increasing vacancy charges and the minimum values correspond to thestable chemical valence (V_O²⁺orV_Y^3-). A new electronic state appears in theelectronic structure of O vacancy, causing that electrons left by a removedneutral O atom are well localized at the vacancy site. However, the presenceof Y vacancy does not create a new state like O vacancy. For divacancies, Y-Yand O-O vacancy pairs are not stable because their binding energies are nearlyzero. However, the Y-O vacancy pair is more stable, with a binding energy of6.97eV. The electron density of the vacancies shows that if the vacancy pairsform, the electrons are well localized between Y and O vacancy sites, likeforming a chemical bond, resulting in very large binding energies of Y-Opairs. Moreover, the Y-O vacancy pairs can attract other vacancies to formsmall stable vacancy clusters with positive binding energies.The relative stability of various He trapping sites is determined by theformation energies. It is found that He prefers to occupy Y vacancy site withlarge volume and low charge density, while it relaxes to the nearest interstitialsite from O vacancy site owing to a high density of O vacancy site and itsrepulsion with full-filled electron shell of He. Moreover, He prefers to move via interstitial site with low migration energy of0.4eV. In the absence ofvacancies, He has a small possibility to cluster themselves as their bindingenergies are very small. However, if Y vacancies are present, they cansignificantly attract He to form He-Vacancy clusters due to a large binding energybetween Y vacancy and He atom. The addition of O vacancies reduces thebinding energies between He and helium-vacancy clusters. The stable structures ofhelium-vacancy clusters show that He prefers to occupy Y vacancies or form adumbbell around the Y vacancy. It is also found that in the presence of Y-O vacancypair, He can occupy O vacancy due to the reduction of density in O vacancy.The above results from ab intio calculations show that Y₂O₃can act assinks for trapping vacancies or He atoms induced by irradiation. If thenano-size Y₂O₃particles are evenly distributed in ferritic steels, they canattract the defects created in matrix under irradiation, thus reduce defectdensity in matrix so as to enhance the anti-irradiation properties of ODSsteels.