The aggregation of defects in gadolinia-doped ceria (GDC) into clusters and larger domains has been studied by atomistic computer simulation. It is found that sub-nanoscale defect clusters prefer a pyrochlore-type structure in which the dopants and vacancies are at next-nearest-neighbor sites, and have a tendency to grow into larger clusters. It was determined that, as defect clusters grow into nano-scaled domains, the C-type rare earth structure, in which the dopants and vacancies are at nearest-neighbor sites, becomes more stable. Simulations suggest that nanodomains serve as the precursor of phase separation and can be easily formed during synthesis. It is believed that doping concentration limits the size of the nanodomains, and this causes GDC to favor small pyrochlore-type clusters at lower concentrations, but C-type nanodomains at higher concentration. Because of this transition, GDC is expected to show initially an increase in conductivity and then a decrease with increasing doping concentration. The lattice parameter of GDC should also show the same trend and could be used as an indicator of the predominant defect structure. The cation mobility is believed to be another important factor limiting the size of defect clusters, and can be used to control the formation of nanodomains during synthesis and thereby improve the electrolyte performance.
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