RELAXOR AND SUPERPARAELECTRIC BEHAVIOR IN THE DISORDERED FERROELECTRICS KLT AND KTN

The relaxor behavior is characteristic of a number of mixed ferroelect ric systems. Early explanations attributed it to a diffuse phase trans ition. Although all relaxors identified so far were lead-based systems , we have recently showed that K1-xLixTaO3 (KLT) also exhibits the rel axer behavior. Moreover, we have proposed a microscopic model to accou nt for this behavior. In parallel, we have also studied a closely rela ted system, KTa1-xNbxO3 (KTN). KTN is structurally similar to KLT but displays a much weaker relaxer behavior and is better described as a s uper-paraelectric. In the present paper we review the microscopic rela xer model and show that it reproduces the essential frequency dispersi on observed in the dielectric constant of KLT. We then compare the res ults of dielectric and polarization hysteresis loop measurements made on KLT and KTN and explicit the relationship that exists between both types of measurements. Finally, we explain the significant differences that exist between the two systems, making KLT a hard relaxor and KTN a soft one. These differences are essentially based on the site and m agnitude of the dipole moments associated with the off-center Li and N b ions in the two systems respectively.