Effect of defect field on dielectric ageing of lead magnesium niobate-lead titanate relaxor ferroelectrics

The ageing effect on the temperature dependence of the dielectric permittivity and loss angle tangent was examined for Pb(Mg_1/3Nb_2/3)_0.9Ti_0.1O₃ and 0.1 wt% MnO-Pb(Mg_1/3Nb_2/3)_0.95Ti_0.05O₃ ceramics. A Debye-like dielectric relaxation process was clearly observed in the higher temperature range for the latter composition, but not for the former. It is stressed that the metastability of the ferroelectric domain structure is due to the presence of defect dipoles in perovskites. Compared with the dielectric poling behaviour of the compositions, a pin effect was introduced to analyze the differences and similarities of reorientation and alignment of microdomain with macrodomain. Defect dipoles are formed from pairs of vacancy and positively charged dopant sites and they provide pinning centres to ferroelectric microdomain walls. As a consequence, microdomains re-establish along a certain direction over time, and the microstructure changes from a disordering to an apparent ordering. Thus a perovskite relaxor ferroelectrics with acceptor dopants exhibits a significant time dependent decrease in the complex dielectric response.