Atomistic Analyses of Competition between Site-Selective Segregation and Association of Point Defects at Grain Boundary in Y2O3-Doped ZrO2

The site-selective occupation of point defects, Y3+ ions (Y'(Zr)) and O2- vacancies (V-o), and their associations at a symmetric tilt grain boundary (GB) are studied to understand their competitive contribution to energetically favorable atomic arrangements by using atomistic simulations. It is found that at the GB there are the favorable sites for segregation of an isolated Y'(Zr). and V-o. This indicates that the driving force for the site-selective segregation is present. Moreover, our results of Y'(Zr)-V-o association at the GB show that the lattice energies are very dispersed despite that a second-nearest neighbor (SNN) vacancy to Y'(Zr), is favored for bulk Y2O3-doped ZrO2. The result suggests that the site-selective segregation has significant effects on the favorable point defect arrangement at the GB core, competing with the point defect associations. For more realistic cases, Monte Carlo simulations are performed to reveal favorable atomic arrangements for a high dopant concentration, where point defects are crowded at the GB. The results show that the region of GB segregation can be classified with respect to O2- coordination to cation species; at the GB core the favorable configuration is not necessarily a SNN O2- vacancy relative to Y3+. On the other hand, eight-fold O2- coordination is sustained for Y3+ ions more than 3 A distant from the GB plane. The difference in O2- coordination may play an important role in O2- ionic conductivity at GBs via the energetics for O2- migration.