Isothermal aging of the complex dielectric permittivity in MnO doped lead magnesium niobate: Lead titanate relaxor ferroelectric ceramics

Abstract

Isothermal aging of the complex permittivity was investigated in MnO doped lead magnesium niobate: lead titanate (0.9 PMN: 0.1 PT) relaxor ferroelectric ceramics. Unlike most normal ferroelectrics, both real and imaginary parts of the permittivity deviate strongly from the logarithmic aging law, both curves tending to saturate at longer times. The effect is most pronounced in the imaginary part of the permittivity which approaches a nearly constant value quite quickly whilst the real part continues to age. The results can be explained using a model for the relaxor in which there is a distribution of micro-polar regions of varying size which contribute to the relaxation component of the polarizability. During aging it is postulated that defect diooles induced by the aliovalent MnO doping reorient so as to stabilize the polar vectors in the micro-polar regions. Larger regions stabilize first, removing the longer relaxation time process and leading to a distribution richer in shorter relaxation times. That the K ¹ vs. K ¹¹ plot taken over very long times shows no evidence of intersecting the K ¹ axis at a large K ¹ value suggests that the micro-polar regions still contribute a major part of the polarizability even after very long aging times. This datum is consistent with evidence from microwave and Raman spectroscopy that conventional soft modes are not major contributors to the dielectric response of relaxor ferroelectric.