Probing Dopant Size Effects on Defect Clustering and Vacancy Ordering in Lanthanide-doped Ceria

Dopant size is known to influence oxygen vacancy-mediated conduction pathways and ionic conductivity in doped ceria, yet the underlying atomic-scale mechanisms remain unclear. Here, we combine neutron total scattering and large-scale atomistic simulations to analyze the local defect structures of two representative doped ceria systems: Ce0.8Gd0.2O1.9 (GDC) and Ce0.8Nd0.2O1.9 (NDC). The local structure of GDC, a commercially used ion conductor, is investigated for the first time using neutron total scattering on 160Gd-enriched samples. GDC exhibits fewer defect clusters, with vacancy pairs preferentially aligned along ⟨111⟩ and ⟨110⟩ directions while disfavoring ⟨100⟩ direction within the cubic fluorite structure. The Gd–Gd interactions in GDC help destabilize ⟨100⟩ ordering, promoting a more open defect network that supports efficient oxygen-ion transport. Unlike Gd3+ (1.053 Å in 8-fold coordination with oxygen), the slightly larger dopant Nd3+ (1.109 Å) in NDC promotes a more compact defect configuration, characterized by increased defect clustering and stabilized ⟨100⟩ vacancy alignment due to dominant Nd–vacancy interactions, substantially reducing ionic conductivity. Gd3+ provides an optimal balance of lattice expansion and preserving favorable defect structure for ion transport. These findings provide a mechanistic understanding of dopant-size controlled conduction pathways in lanthanide-doped ceria and fundamentally contribute to the understanding of charge transport by ions, electrons, and protons in next-generation conducting materials.