Non-Classical Electrostriction in Hydrated Acceptor Doped BaZrO₃: Proton Trapping and Dopant Size Effect

Point defects such as oxygen vacancies and protonic interstitials are not only essential for ionic conductivity in oxides since they also affect the mechanical and electromechanical properties. These properties of nominally dry and hydrated proton-conducting BaZr_0.85M_0.15O_2.925+δH_2δ (M = Al, Ga, Sc, In, Y, Eu) ceramics are investigated. Doping decreases Young's modulus with increasing ionic radii difference between the dopant and the host. Nominally dry samples show consistently higher Young's moduli than hydrated samples. All samples exhibit large non-classical electrostriction, with a negative electrostriction coefficient M₃₃<0. M₃₃ shows saturation with the field and a non-ideal Debye relaxation with frequency. The low-frequency M₃₃ value for both dry and hydrated samples shows a similar dependence on dopant radius as Young's modulus. For the hydrated samples, the relaxation frequency increases by a factor >100 in the series Ga-Y, emphasizing the importance of proton trapping, with Y-doped samples having minimal trapping energy. This coincides with the fact that the saturation strain for Y-doped samples is also the smallest. In light of these findings, it is concluded that the present data give strong evidence for the existence of defect-related elastic dipoles in acceptor doped barium zirconate and that the non-classical electrostriction originates in their reorientation under electric field.