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

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The site-selective occupation of point defects, Y³⁺ ions (Y′_Zr) and O²⁻ vacancies (V_\ddot{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_\ddot{O}. This indicates that the driving force for the site-selective segregation is present. Moreover, our results of Y′_Zr-V_\ddot{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 Å 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.