Oxygen defects and low-frequency mechanical relaxation in Pb-Ca and Pb-Sm titanates

The low-frequency behaviour of the isotropic elastic modulus and mechanical loss tangent of modified lead titanate ferro-piezoelectric ceramics have been studied by the three-point bending technique. Measurements of these parameters as a function of the temperature were performed in the frequency range of 0.2 to 20 Hz. The results show anomalies in both parameters at the transition temperature and below this temperature. The anomalies below the transition temperature have wide maxima in mechanical loss whose temperatures increase when the measuring frequency increases. Values for activation energies were obtained that suggest the existence of mechanical relaxation mechanisms where point defects and 90 degrees ferroelectric domains are involved. The main species of point defects should be oxygen vacancies. Comparisons between experimental results and existing theoretical models are made.