Dielectric and mechanoelastic relaxations due to point defects in layered bismuth titanate ceramics

Complex permittivity and Young's modulus provide relevant information on the role of point defects in the dielectric and mechano-elastic properties of ferroelectric materials. Low-frequency measurements as a function of the temperature performed on Bi 4 Ti 3 O 12 (BIT) have shown that point and dipole defects are frozen close to domain walls. Low-temperature dipole defect relaxation processes take place with characteristic times (τ 0 ) of the order of 10 -11 s and 10 -12 s and activation energies ( E a ) of 0.70 eV and 0.65 eV for dielectric and mechano-elastic relaxations, respectively. At higher temperatures new dielectric relaxation peaks appear that can be attributed to jumps of de-iced oxygen vacancies (τ 0 ≅10 -11 s, E a = 1.08 eV, T ≅300 °C) and to vacancy migration (τ 0 ≅10 -15 s, E a = 1.90 eV, T ≅450 °C). Elastic relaxation peaks are also present close to 300 °C whose activation energy (1.50 eV) and characteristic time (10 -15 s) suggest a vacancy migration process. Close to 500 °C with E a = 2.30 eV and τ 0 ≅10 -17 s another relaxation peak, which should correspond to domain wall viscous motion near the phase transition temperature, is observed. The Young's modulus has a smooth step at T ≅300 °C that we attribute to a change in the mobility of oxygen vacancies with respect to the domain walls. Below 300 °C the vacancies are frozen in the domain walls and they are de-iced and distributed throughout the material at temperatures above 300 °C. The experimental results show that the material is softer when the vacancies are linked to domain walls than when they are distributed throughout the material. The diffusion of vacancies back to the domain wall traps at room temperature takes a long time (days).