Institute of High Pressure Physics

Despite seven decades of research on the unusual dielectric properties of relaxor ceramics in the wide surrounding of the paraelectric-ferroelectric transition, a fundamental model explaining and coherently describing this phenomenon remained a challenge. There are: specific 'diffused' changes in the 'dielectric constant' near the 'para-ferro' transition, extraordinary sensitivity to the external electric field ('tunability') and the so-called 'glassy' dynamics.

These subjects are addressed in a recent publication by the X-PressMatter/NL10 team [S.J. Rzoska et al. Critical insight into pretransitional behavior and dielectric tunability of relaxor ceramics. Materials 16 (2023) 7634 that is fully Open Access]. Authors propose a qualitatively new model for these systems. The work also includes an experimental test using the 'distortions-sensitive analysis'.

Fig. ‘Dielectric constant’ in  Ba_0.65Sr_0.35TiO₃ relaxor ceramic and the ‘classic’ Curie-Weiss (CW) equation focused analysis, with the Burns temperature T_B.

In the new approach, a weakly discontinuous para-ferro phase transition takes place in relaxor ceramics, as a result of limiting the growth of precritical fluctuations to the size of the ceramic grains. However, during the development of the pre-ferro phase within the grains, strong local electric fields are also spontaneously created, which must be coupled with the order parameter (electric polarization) for this class of materials. This causes the bias from the Curie-Weiss behavior, which is characteristic of dielectric constants in ‘homogeneous’ ferroelectrics, and pseudospinodal changes in the correlation length of fluctuations, matched with a sequence of singular temperatures. In relaxor ceramics, this leads to ‘diffused’ changes in 'dielectric constant', and the 'tunability' results from the interaction of spontaneous electric fields between the grains with the external electric field. The mentioned factors also result in the so-called frustration, which must lead to the 'glassy' dynamic. However, it is not described by the VFT relation as has been suggested so far, because it is influenced  by the local uniaxiality.

The mentioned work is not only a new and simple picture of the phenomenon including all (!) experimental properties. It is also a coherent set of equations for their descriptions and for supporting the design in materials engineering. The latter is particularly important due to innovative applications ranging from varactors to electric field-tunable antennas or the electrocaloric effect for heating and cooling even on a 'micro' scale.

This work also shows that the heuristic concepts of Burns temperature and 'polar nanoregions (PNRs)' - the dominant models/explanations to date - are not needed.