Chemical expansion and its dependence on the host cation radius

The defect-induced lattice expansion (chemical expansion), and corresponding relaxation patterns around oxygen vacancies, were examined as a function of host cation radius for HfO2, ZrO2, CeO2, UO2, ThO2, and Bi2O3 fluorite-structured oxides. Analysis of data from the literature, combined with new molecular dynamics simulations, found a maximum in the effective radius of an oxygen vacancy (related to the lattice contraction around a vacancy) for a host cation size close to that of cerium. In other words, ceria shows the highest chemical expansion, whereas the other studied materials, with either smaller or bigger host cations than Ce, undergo smaller chemical expansion. Significant asymmetric lattice relaxation around a vacancy for smaller cations and 2nd nearest neighbor cation relaxations around a vacancy for larger cations play a strong role in forming the maximum. The impact of this vacancy relaxation on ionic conductivity is discussed, and through careful analysis of the vacancy-anion radial distribution functions, an estimate of a critical vacancy concentration (c* = 0.025%), above which vacancy interactions exist, was derived.