Electromechanical coupling in polaronic ceria

Oxygen-defective metal oxides like cerium oxides exhibit giant electrostriction and field-induced piezoelectricity due to a dynamic electrosteric interplay between oxygen defects, 𝑉^∙∙_𝑂, and the fluorite lattice. While such mechanisms are generally attributed to oxygen vacancies, recent results also highlight that trapped cationic defects, 𝐶𝑒′_𝐶𝑒, i.e., small polarons, can contribute to the electromechanical properties of ceria. Here, we study nanocrystalline 5% Ca- and 10% Gd-doped ceria thin films with a high density of point defects and a constant oxygen vacancy concentration at 5 % molar. We deposit thin films at low temperatures to promote microstructure disorder, i.e., nano-crystallinity, where the oxygen vacancies have low mobility due to high grain boundary interface densities. Still, the Ca²⁺ and Gd³⁺ dopants' sizes and valence differences modulate trapping effects toward the defects in the lattice, giving an insight into the electromechanical nature of the defects in the material dominating the electrostriction. We find that electrosteric dopant-oxygen vacancy interactions only slightly affect the electromechanical properties, which mainly depend on the frequency and intensity of the applied electric field. On the other hand, n-type polaron, 𝐶𝑒′_𝐶𝑒, transport can emerge below the breakdown limit. These effects lead to an electromechanical coupling with a longitudinal electrostriction coefficient, 𝑀₃₃, above 10^-16 V2m^-2. Our results suggest that polaronic mechanisms substantially contribute to the electromechanical coupling in ceria. Also, the large ionic radius difference between Ce³⁺ and Ce⁴⁺ induces a large electro-strain upon polaron hopping, coupling electric stimuli to the observed electrostriction. This analysis provides new insights into the electromechanical effect of small polaronic semiconductive materials, opening new designing criteria for efficient electromechanical energy conversion.