A New Understanding of Chemical Expansion in Non-Stoichiometric Cerium Oxide with Elastic Force Dipole

In this work, a new continuum model, informed by DFT simulations, is developed to predict the chemical expansion observed in non-stoichiometric oxides and is applied to study the expansivity of oxygen vacancies in CeO₂. The chemical expansion is a summation of two competing processes: the formation of oxygen vacancy and the change in cation radius. We introduce an elastic dipole tensor to determine the elastic energy per defect introduced to the system around the oxygen vacancy. We show that this tensor, which can be accurately predicted from first-principle DFT calculations, can be used to predict the chemical expansion of ceria as well as other fluorite structure oxides. Compared to previous work which obtains expansivity based on empirical potentials, our work provides an efficient way of computing it directly by DFT calculations. Furthermore, we discuss how the elastic dipole tensor can predict the O₂ partial pressure vs O/Ce ratios in strained systems and show that CeO₂ can be reduced more easily in the presence of tensile strains.