Revision of the freezing concept in relaxor ferroelectrics: the case of Na0.5Bi0.5TiO3-Sr0.7Bi0.2TiO3 solid solutions

Abstract

Presence of a frequency-independent maximum and a frequency-dependent shoulder in temperature-frequency dependence of dielectric permittivity are characteristic features of NBT-based solid solutions, while interpretation of these features is insufficient. In the present work, dielectric dispersion is studied in a wide frequency range (20 mHz–30 GHz) in (1-x)NBT-xSr0.7Bi0.2TiO3 solid solutions, in which a continuous transfer from NBT-like to pure relaxor-like behaviour is observed upon increasing of Sr0.7Bi0.2TiO3 concentration. Temperature dependence of dielectric dispersion and static dielectric permittivity in different compositions of this solid solutions group is analysed. The observed behaviour of static dielectric permittivity upon increasing of x is interpreted as a diminishing of transition between the low and the high-temperature states, while the character of the dielectric dispersion remains the same in both sides of the transition. Upon increasing of x, the dielectric dispersion transforms from the behaviour characteristic to relaxor ferroelectrics into the behaviour resembling dipolar glass. Problems, arising from introducing of the concept of freezing, described by the Vogel-Fulcher law, are discussed.