XRD and Molecular Dynamics Insights into Lattice Behavior of Oxide Nanocatalysts: The Case of CeO₂

Nanocrystalline CeO₂ exhibits size-dependent lattice distortions linked to defect chemistry and surface effects. However, the relationships between the oxidation state, surface interactions, and nanoparticle structure remain unclear in the existing literature, particularly when inferred from conventional nanoparticle diffraction techniques, including powder X-ray diffraction. As a result, the atomistic origin of lattice expansion or contraction with the crystallite size of ceria nanoparticles is still debated. Here, synchrotron X-ray powder diffraction data are analyzed using Rietveld refinement supported by advanced peak profile modeling based on whole powder pattern modeling (WPPM), including thermal diffuse scattering (TDS). The latter provides direct access to information on lattice dynamics. Indeed, we simultaneously determine the size distributions of crystalline domains and their atomic displacements, which are then compared and quantitatively validated with molecular dynamics (MD) simulations. Reactive MD simulations further reveal that vacancy-rich surfaces induce lattice contraction at small particle sizes under vacuum, whereas water adsorption causes surface hydroxylation and lattice expansion. These results explain lattice parameter variations in nanocrystalline ceria through the interplay of surface chemistry and environment. This insight is critical for the correct interpretation of diffraction-derived structural parameters in oxide nanocatalysts used in redox and oxygen storage applications.