The role of point defects in the mechanical behavior of doped ceria probed by nanoindentation

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. Pr ³⁺- 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. Nanoindentation measurements performed on ceria doped with Pr⁴⁺, Pr³⁺, Lu³⁺, Gd ³⁺ demonstrate that the rearrangement of point defects may be a major source of creep at room temperature. Nanoindentation is shown to be an effective technique for identifying materials with labile point defects, which may point to practical functionality such as high ionic conductivity, large electrostriction, and inelasticity.