Invited
To be or not to be antiferroelectric

The study of antiferroelectrics has undergone a revival during the last years due to the possibility to use these materials as energy-storage and solid-state-cooling elements. This has prompted new efforts to understand the nature of the antiferroelectricity, but some new findings, instead of clarifying the phenomenon, has posed more questions about its true nature (coexistence with ferroelectricity, new ground states, possible ferrielectricity…). Due of this, it is necessary to understand better archetype materials, as PbZrO­3 and its related solid solutions. These materials show antiferroelectric behaviour, but they have also energetically available ferroelectric phases. The main lattice instabilities (several soft modes of different symmetries are present in PbZrO­3) compete and couple in a way that allows to trigger different phases with disparate dynamics, mainly when Zr is substituted with other cations.

The intermediate polar or antipolar states found experimentally depend on the thermodynamics and the energetic balance during the heating or cooling process in experiments, and they can be affected by many extrinsic factors (cooling rate, thermal history, pressure, stress) as well as intrinsic ones (defects or ionic substitutions in the crystal lattice). The study of phase transitions using phonon spectroscopies brings important information about the lattice instabilities that trigger the sequence of the phase transitions, either as primary actors, or through coupling with other order parameters; and may help to unify the microscopic picture of the (anti)ferroelectricity with its macroscopic manifestation.

I will discuss Raman scattering in pure PbZrO3, as its antiferroelectric Pbam phase has been challenged recently as the true ground state; and also in substituted PbZrO3 materials, where intermediate phases are present and easily identified by this technique, showing that their dynamics and stability strongly depend on the defects present in the samples and the experimental conditions