On the origin of the lattice constant anomaly in nanocrystalline ceria

The lattice parameter of nanocrystalline ceria films prepared by sputtering was monitored as a function of annealing temperature. Within the temperature range of 150-420 °C, an equilibrium with atmospheric oxygen is established within a few hours, whereas grain growth does not occur. On the basis of the experimental results and analysis of literature data, we present a model that posits the formation of a non-uniform grain structure with stoichiometric interiors and oxygen deficient boundaries. This model, based on defect thermodynamics, correctly describes the dependence of the lattice parameter of nanocrystalline ceria on annealing temperature and grain size and can be extended to other materials as well.