Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R2O3 (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO2. Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.