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. Counter intuitively, 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 Ce+3 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 angstrom rather than a 3.89 angstrom 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 0 anion opposite vacancy, thereby expanding ceria (Ce-vac(4+)-Ce-vac(4+) distance of 4.14 angstrom). The relaxation of Ce+4 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.