Phase transitions in perovskite-type relaxor ferroelectrics

Abstract

Relaxor ferroelectrics (relaxors) form a special class of ferroelectric materials of which the understanding remains a challenging problem.Most technologically important relaxors crystallize in the so-called ABO3 perovskite-type structure; between those PbMg1/3Nb2/3O3 (PMN) and Na1/2Bi1/2TiO3 (NBT) which are considered as model relaxor ferroelectrics.From an application point of view, relaxor-based materials have been reported to exhibit near structural phase boundaries outstanding electromechanical properties, which point to a potential revolution in electromechanical transduction for a large range of applications.As a consequence, the potential impact of thin-film relaxor ferroelectrics has stimulated a fast growing interest.Although it has been realized that strain effects at the film-substrate interface modify dramatically their physical properties, there is incomplete understanding of the responsible mechanisms.In this presentation we will mainly focus on two innovative approaches towards the understanding of phase transitions of the relaxors PMN and NBT: on the one hand temperaturedependent birefringence imaging and on the other hand first-time high-pressure investigations of relaxor ferroelectrics by Raman spectroscopy, diffraction and diffuse scattering.As a matter of fact, the observed pressure-dependent transitions are very unusual and point, among other things, to new relaxorspecific spectral signatures and to important pressure-induced changes of the local structure and order.We further show that an external pressure of several GPa, as can be met in thin films, alters fundamentally the structural and polar properties in relaxor ferroelectrics, suggesting that intrinsic instabilities towards pressure play an important role in the unwished reduction of dielectric properties in application-designed relaxor thin films.