No abstract

The local crystal and domain structures of K0.5Bi0.5TiO3 ceramics were investigated by transmission electron microscopy (TEM) and selected‐area electron diffraction (SAED). The individual grains showed a lamellar domain structure, and on the basis of spot splitting along the characteristic crystallographic directions, 90°a–a‐ and 90°a–c‐type domains were identified. Furthermore, lamellar features within the 90° domains were observed, which we presumed were 180° domains; however, in the case of P4mm structures they cannot be distinguished by SAED. The 90° domain boundaries were (011) and (101) twin planes, typical for tetragonal perovskites. The domains could be designated as rotation twins, where the symmetry element describing the relationship between two twin domains is a twofold twin axis [011]/[101] and the symmetry operation is a 180° rotation. In order to determine the dynamics and the temperature of the phase transformation from the tetragonal to the cubic structures, high‐temperature XRD and in situ heating TEM analyses were performed. The results showed a gradual phase transformation in the temperature range from ∼280° to ∼450°C, where some grains lost their polar domains at a lower temperature than others. These findings confirmed the existence of a binary stability field in which the cubic and tetragonal structures coexist.

The polarization and strain response of ferroelectric materials at fields below the macroscopic coercive field is of a paramount importance for the operation of many electronic devices. The response of real ferroelectric and related materials is in general complex and difficult to interpret. The reason for this is that many processes in a ferroelectric material contribute to its properties, often concurrently. Examples include motion of ferroelectric and ferroelastic domains, presence of domains within domains, dynamics of different types of polar nano-entities, interaction of polar nano-entities (e.g., polar nanoregions in relaxors) with the strain and polarization within domains, motion of defects and rearrangement of defect clusters and their interaction with polarization and strain. One signature of these processes is nonlinearity of the strain and polarization. Most ferroelectrics exhibit nonlinear response at all practical field levels meaning that the apparent material coefficients depend on the amplitude of the driving excitation. In this paper we show that an investigation of nonlinear behavior is a sensitive way to study various mechanisms operating in dielectric and piezoelectric materials. We review the basic formalism of the nonlinear description of polarization and strain, give a physical interpretation of different terms and illustrate this approach on numerous examples of relaxors, relaxor ferroelectrics, hard and soft ferroelectrics, and morphotropic phase boundary compositions. An experimental approach based on a lock-in technique that is well-suited for such studies is also discussed. I. INTRODUCTION Modern technology demands improved functionality, reliability and tolerances of electronic components. Dielectric and piezoelectric nonlinearities present opportunities and challenges in this regard. From the application point of view, nonlinearities can be desirable, for example in tunable filters and antennas, positive

Lead-based relaxor ferroelectrics are characterized by outstanding piezoelectric and dielectric properties, making them useful in a wide range of applications. Despite the numerous models proposed to describe the relation between their nanoscale polar structure and the large properties, the multiple contributions to these properties are not yet revealed. Here, by combining atomistic and mesoscopic-scale structural analyses with macroscopic piezoelectric and dielectric measurements across the (100-x)Pb(Mg 1/3 Nb 2/3) O 3-xPbTiO 3 (PMN-xPT) phase diagram, a direct link is established between the multiscale structure and the large nonlinear macroscopic response observed in the monoclinic PMN-xPT compositions. The approach reveals a previously unrecognized softening effect, which is common to Pb-based relaxor ferroelectrics and arises from the displacements of low-angle nanodomain walls, facilitated by the nanoscale polar character and lattice strain disorder. This comprehensive comparative study points to the multiple, distinct mechanisms that are responsible for the large piezoelectric response in relaxor ferroelectrics.

No abstract