THE SPONTANEOUS RELAXOR-FERROELECTRIC TRANSITION OF PB(SC0.5TA0.5)O3

A zero-field spontaneous relaxor-ferroelectric transition is reported in Pb(Sc0.5Ta0.5)O3 (PST). This behavior is different from that of oth er relaxors, where such transitions occur only under the field. A high ly disordered PST that has the wide relaxation spectrum typical of rel axors is shown to transform spontaneously into a macroscopic ferroelec tric state. Introduction of defects (lead vacancies) into the material impedes the transition resulting in the usual relaxor behavior. Diele ctric properties of PST, with and without defects, are analyzed. For t he interpretation of the observed properties, a model invoking an addi tional nonpolar phase is proposed. This model does not imply a freezin g in the system. At the low-frequency limit, it is possible to account for the Vogel-Fulcher (VF) law for the temperature of the maximum of the dielectric constant, using only the commonly accepted assumption o f an exponentially wide relaxation time spectrum that shrinks on heati ng. The presented approach interprets the observed proximity between t he ferroelectric phase transition temperature and that of the freezing temperature obtained from the VF relation.