Structure and segregation of dopant-defect complexes at grain boundaries in nanocrystalline doped ceria

Grain boundaries (GBs) dictate vital properties of nanocrystalline doped ceria. Thus, to understand and predict its properties, knowledge of the interaction between dopant-defect complexes and GBs is crucial. Here, we report atomistic simulations, corroborated with first principles calculations, elucidating the fundamental dopant-defect interactions at model GBs in gadolinium-doped and manganese-doped ceria. Gadolinium and manganese are aliovalent dopants, accommodated in ceria via a dopant-defect complex. While the behavior of isolated dopants and vacancies is expected to depend on the local atomic structure at GBs, the added structural complexity associated with dopant-defect complexes is found to have key implications on GB segregation. Compared to the grain interior, energies of different dopant-defect arrangements vary significantly at the GBs. As opposed to bulk, the stability of oxygen vacancy is found to be sensitive to the dopant arrangement at GBs. Manganese exhibits a stronger propensity for segregation to GBs than gadolinium, revealing that accommodation of dopant-defect clusters depends on the nature of dopants. Segregation strength is found to depend on the GB character, a result qualitatively supported by our experimental observations based on scanning transmission electron microscopy. The present results indicate that segregation energies, availability of favorable sites, and overall stronger binding of dopant-defect complexes would influence ionic conductivity across GBs in nanocrystalline doped ceria. Our comprehensive investigation emphasizes the critical role of dopant-defect interactions at GBs in governing functional properties in fluorite-structured ionic conductors.