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: Ce_0.8Gd_0.2O_1.9 (GDC) and Ce_0.8Nd_0.2O_1.9 (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.