Abstract:The influence of dopant size and oxygen vacancy concentration on the room temperature elastic modulus and creep rate of ceria doped with Pr⁴⁺, Pr³⁺, Lu³⁺, and Gd³⁺, is investigated using a nanoindentation technique. Measurements are conducted with both fast (15 mN s⁻¹) and slow (0.15 mN s⁻¹) loading modes, including a load-hold stage at 150 mN of 8 s and 30 s, respectively. Based on the data obtained using the fast loading mode, it is found that: 1) the dopant size is a primary determinant of the elastic modulus—the larger dopants (Pr³⁺ and Gd³⁺) produce lower unrelaxed moduli which are independent of the oxygen vacancy concentration. 2) The rearrangement of point defects is the major source of room temperature creep observed during load-hold. Pr³⁺- and Gd³⁺-doped ceria display the higher creep rates: due to their large size, they repel oxygen vacancies (V_O), thereby promoting the formation of O₇–Ce_Ce–V_O complexes that are capable of low temperature rearrangement. Lower creep rates are observed for Pr⁴⁺- and Lu³⁺-doped ceria: the former has no vacancies and the latter, immobile vacancies. 3) Nanoindentation is a practical technique for identifying materials with labile point defects, which may indicate useful functionality such as high ionic conductivity, large electrostriction, and inelasticity.