Re-Evaluating CeO₂ Expansion Upon Reduction: Noncounterpoised Forces, Not Ionic Radius Effects, Are the Cause

Ceria (CeO₂) is widely used in reduction and oxidation processes such as catalysis, solid-oxide fuel cells and electrolyzers, and thermochemical redox processes. Counterintuitively, as ceria reduces and oxidizes, it expands and contracts, respectively. This has been attributed to the larger ionic radius of Ce³⁺ as compared with Ce⁴⁺. However, electronic structure calculations (DFT+U) detailed herein show that this is incorrect. While the presence of Ce³⁺ cations causes local expansion of their coordinating O anions, the expansion is compensated by the contraction of the O-Ce bonds in the second coordination shell. This results in only negligible changes in the Ce sublattice (Ce³⁺-Ce⁴⁺ distances of 3.90 Å rather than a 3.89 Å Ce⁴⁺-Ce⁴⁺ distance in oxidized ceria). The severing of Ce-O bonds upon the formation of an O vacancy results in noncounterpoised forces acting on the vacancy neighboring Ce cations, which relax toward the O anion opposite the vacancy, thereby expanding ceria (Ce⁴⁺ _vac-Ce⁴⁺ _vac distance of 4.14 Å). The relaxation of Ce⁴⁺ cations away from the vacancy rather than toward the vacancy, as is found in other materials, arises because ceria reduction results in the population f orbitals rather than d-O p antibonds. The corrected explanation for ceria expansion presented here will enable better design of ceria-based systems and modifications to ceria, such as doping, that will improve its performance.