FREEZING OF THE POLARIZATION FLUCTUATIONS IN LEAD MAGNESIUM NIOBATE RELAXORS

The dielectric relaxation of a solid solution of 10-mol % lead titanate in lead magnesium niobate is found to be similar to the magnetic relaxation in spin-glass systems.1-3 Based on this analogy, it is proposed that the relaxor ferroelectric is a polar-glassy system which has thermally activated polarization fluctuations above a static freezing temperature. An activation energy and freezing temperature of 0.0407 eV and 291.5 K, respectively, were found by analyzing the frequency dependence of the temperature of the dielectric maximum using the Vogel-Fulcher relationship. 4,5 It has also been shown that a macroscopic polarization is sustained on heating up to this freezing temperature. A coupling between nanometer scale clusters is believed to control the kinetics of the fluctuations and the development of a frustration as the system freezes into states of local equilibrium. The possibility of an orientational freezing associated with the ferroelastic nature of the nanosized polar regions in the rhombohedral relaxor families as well as a polar freezing is discussed. A diffuse phase transformation is believed to arise due to a dispersion in the fluctuation frequency of the polarization. A qualitative model for the relaxation time spectrum is also proposed in which the width of the spectrum broadens strongly near the freezing temperature.