Abstract :
A solid solution of 10 mol% lead titanate in lead magnesium niobate (PMN) was prepared by the conventional dry technique. Different heat treatments were chosen to obtain ceramic specimens with three different average grain sizes, namely 4, 7, and 11 μm. Dielectric spectroscopy was carried out (in the frequency range 100–300 kHz) to study the temperature and frequency dependence of the relaxational behaviour. The temperature Tm of the observed maxima in the dielectric permittivity was fitted to the probing frequency through the Vogel–Fulcher equation. It is found that the ‘freezing’ temperature decreases, and the activation energy increases, with increasing average grain size. It is also observed that for each specimen there is a typical frequency, above which the Vogel–Fulcher equation does not provide a very good description of the cluster dynamics. For these higher frequencies, there is a clear change in the value of the activation barrier, and also a qualitative change in the nature of the cluster dynamics, which is better described by a model proposed by Cheng et al. [J. Appl. Phys. 79 (1996) 8615]. This crossover in relaxational behaviour is also observed for pure (i.e., undoped) PMN.
