B. Jimenez et al., Dielectric and mechanoelastic relaxations due to point defects in layered bismuth titanate ceramics, J PHYS-COND, 13(33), 2001, pp. 7315-7326
Complex permittivity and Young's modulus provide relevant information on th
e role of point defects in the dielectric and mechano-elastic properties of
ferroelectric materials. Low-frequency measurements as a function of the t
emperature performed on Bi4Ti3O12 (BIT) have shown that point and dipole de
fects are frozen close to domain walls. Low-temperature dipole defect relax
ation processes take place with characteristic times (tau (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 high
er temperatures new dielectric relaxation peaks appear that can be attribut
ed to jumps of deiced oxygen vacancies (tau (0) congruent to 10(-11) s, E-a
= 1.08 eV, T congruent to 300 degreesC) and to vacancy migration (tau (0)
congruent to 10(-15) s, E-a = 1.90 eV, T congruent to 450 degreesC). Elasti
c relaxation peaks are also present close to 300 degreesC whose activation
energy (1.50 eV) and characteristic time (10(-15) s) suggest a vacancy migr
ation process. Close to 500 degreesC with E-a = 2.30 eV and tau (0) congrue
nt to 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 congruent to 300 degreesC that we a
ttribute to a change in the mobility of oxygen vacancies with respect to th
e domain walls. Below 300 degreesC the vacancies are frozen in the domain w
alls and they are de-iced and distributed throughout the material at temper
atures above 300 degreesC. 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).