  Cerium oxide (ceria, CeO2) is one of the most promising mixed ionic and
electronic conducting materials. Previous atomistic analysis has covered widely
the effects of substitution on oxygen vacancy migration. However, an in-depth
analysis of the role of cation substitution beyond trivalent cations has rarely
been explored. Here, we investigate soluble monovalent, divalent, trivalent and
tetravalent cation substituents. By combining classical simulations and quantum
mechanical calculations, we provide an insight into defect association energies
between substituent cations and oxygen vacancies as well as their effects on
the diffusion mechanisms. Our simulations indicate that oxygen ionic
diffusivity of subvalent cation-substituted systems follows the order Gd>Ca>Na.
With the same charge, a larger size mismatch with Ce cation yields a lower
oxygen ionic diffusivity, i.e., Na>K, Ca>Ni, Gd>Al. Based on these trends, we
identify species that could tune the oxygen ionic diffusivity: we estimate that
the optimum oxygen vacancy concentration for achieving fast oxygen ionic
transport is 2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800
K. Remarkably, such a concentration is not constant and shifts gradually to
higher values as the temperature is increased. We find that co-substitutions
can enhance the impact of the single substitutions beyond that expected by
their simple addition. Furthermore, we identify preferential oxygen ion
migration pathways, which illustrate the electro-steric effects of substituent
cations in determining the energy barrier of oxygen ion migration. Such
fundamental insights into the factors that govern the oxygen diffusion
coefficient and migration energy would enable design criteria to be defined for
tuning the ionic properties of the material, e.g., by co-doping.
