Invited
Understanding domain-wall dynamics from macroscopic nonlinearities and multiscale structural analysis

Dielectric and piezoelectric nonlinearity and hysteresis in ferroelectric ceramics are commonly explained by the displacements of domain walls or similar interfaces, translating through the crystal lattice across randomly distributed pinning centres. Such a mechanism of domain wall motion is described by the Rayleigh relations, although real examples of ferroelectric materials tend to deviate from the model, for example in relaxor ferroelectrics with the so-called low-angle domain walls, or in ferroelectrics with conductive domain walls such as those in BiFeO3. It was shown that in Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–PT) relaxor ferroelectrics the intrinsic charge disorder on the B sites (Nb5+,Mg2+) of the perovskite crucially affects the structure and properties of these materials at all length scales and contributes to exceedingly large nonlinearities and hysteresis. When compared to the classical soft PZT ceramics, the piezoelectric responses of both systems closely fit to the Rayleigh relations but only at their morphotropic phase boundaries (MPBs); deviations from Rayleigh behaviour are observed as the composition moves away from the MPB in both the tetragonal and rhombohedral/monoclinic compositions. While off-MPB compositions of PZT with lower PT contents tend to show underdeveloped hysteresis loops, the response of monoclinic PMN-PT becomes softer with strongly nonlinear and hysteretic weak-field responses. Rayleigh-like responses were also observed in tetragonal PMN-PT compositions upon heating, as well as in the PMN-29PT ceramics when doped with samarium.

In this contribution, we will present our recent results on the nonlinear converse piezoelectric and dielectric characterization of PMN-PT, Sm-doped PMN-29PT and donor-doped (soft) PZT over a wide range of chemical compositions, electric fields, frequency and temperature. In link with the multiscale structural characterisation of these materials we will demonstrate the key role of domain-wall dynamics to the responses and discuss possible reasons for the convergence of the response towards the Rayleigh-type in all analysed materials’ systems.