Mechanism of diffuse phase transition in relaxor ferroelectrics

Dielectric properties of lead magnesium niobate (PMN) and Ta-bearing strontium barium niobate (SBNT) were measured as a function of temperature, and the crystal structure of PMN was refined by the Rietveld analysis method. The results of structure refinement indicate that the volume of polar microregions (PMR) increases with decreasing temperature. The dielectric properties of PMN and SBNT are well explained by an advanced theory of dielectric dispersion. The anomalous behavior in dielectric permittivity ( ε ) observed on the low temperature side of T m , the temperature of ε maximum, is explained by simple dielectric relaxation, while on the high-temperature side of T m is explained by the volume increase of PMR. It is concluded that these is no phase transition around T m , and the diffuse phase transition is an overlapping phenomenon of volume increase of PMR, freezing process of fluctuating dipoles in PMR and dielectric dispersion around the measuring frequrncy. A model of diffuse phase transition and relaxor ferroelectrics is proposed. The difference between normal ferroelectrics and relaxor ferroelectrics is discussed from the viewpoints of spreading of soft-mode phonons and disorder in the crystals.