MECHANISM OF DIFFUSE PHASE-TRANSITION IN RELAXOR FERROELECTRICS

Dielectric properties of lead magnesium niobate (PMN) and Ta-bearing s trontium barium niobate (SBNT) were measured as a function of temperat ure, and the crystal structure of PMN was refined by the Rietveld anal ysis method. The results of structure refinement indicate that the vol ume of polar microregions (PMR) increases with decreasing temperature. The dielectric properties of PM N and SBNT are well explained by an a dvanced theory of dielectric dispersion. The anomalous behavior in die lectric permittivity (epsilon) observed on the low temperature side of T(m), the temperature of epsilon maximum, is explained by simple diel ectric relaxation, while on the high-temperature side of T(m) is expla ined by the volume increase of PMR. It is concluded that these is no p hase transition around T(m), and the diffuse phase transition is an ov erlapping phenomenon of volume increase of PMR, freezing process of fl uctuating dipoles in PMR and dielectric dispersion around the measurin g frequrncy. A model of diffuse phase transition and relaxor ferroelec trics is proposed. The difference between normal ferroelectrics and re laxor ferroelectrics is discussed from the viewpoints of spreading of soft-mode phonons and disorder in the crystals.