From spheres to rods: tuning defects and dislocations generated by oxygen disorder in CeO₂ nanoparticles

Abstract

CeO₂ is key in catalysis, fuel cells, chemical mechanical planarization, and oxygen storage due to its Ce⁴⁺ / Ce³⁺ redox couple and oxygen mobility. At the nanoscale, properties become morphology-dependent, yet the link between morphology and mechanical behavior‒particularly elastic properties and defect structure‒remains underexplored. Here we investigate four CeO₂ morphologies (nanoparticles, spherulites, intermediate rods, and long rods) using XRD line profile analysis, bond valence sum (BVS) calculations, and DFT+U. Rietveld-refined lattice parameters serve as direct DFT inputs. Four Williamson-Hall models quantify crystallite size, microstrain, dislocation density, and character, with DFT-computed direction-dependent Young’s moduli and contrast factors. Results show bulk (≈169 GPa), shear (≈58.8 GPa), and Young’s (≈158 GPa) moduli remain constant within uncertainty across morphologies. The high dislocation densities (up to 10¹⁵ m⁻²) represent a small volume fraction (≈0.03 %) and do not appreciably alter the effective polycrystalline moduli. In contrast, defect structure varies dramatically: dislocation density increases from 10¹¹ to 10¹⁵ m⁻², transitioning from screw-dominated to mixed / edge character with aspect ratio. Oxygen displacement parameters (Uiso, O = 0.0136 – 0.0669 A²) peak in rods, indicating enhanced anionic disorder, while BVS (4.71– 4.75 v.u.) reveals compressed Ce-O bonds. These findings demonstrate that defect populations are morphology-dependent, enabling independent optimization for catalysis, chemical mechanical planarization (CMP), solid oxide fuel cells (SOFCs), and coatings, while the mechanical backbone of CeO2 remains unchanged.