Antiferroelectrics, long considered the less exciting cousin of ferroelectrics, are having a moment. These materials, which possess antipolar order (i.e., antiparallel alignment of polarization at zero field), can be switched to polar (parallel) order by an external electric field thus producing a reversible antiferroelectric-to-ferroelectric phase transition and a characteristic double polarization-electric-field hysteresis loop. Such a unique field-induced antipolar-to-polar transition endows antiferroelectrics with properties that are of great interest for a range of applications including nonlinear dielectrics, capacitive-energy storage, electrothermal-energy conversion, and electromechanical actuation. Compared to their ferroelectric relatives, they are considerably less well studied and understood. Here, we apply the lessons of thin-film epitaxy to the study of antiferroelectrics. This talk will provide an overview of our recent efforts to synthesize, control, and study antiferroelectric perovskite oxides. We will focus on classic, prototypical antiferroelectric materials (PbZrO₃ and PbHfO₃ and solid solutions and multilayers derived from these parent materials) and demonstrate how epitaxy, strain, and buffer layers can allow us to finely control the orientation of the resulting orthorhombic films and, in turn, how this orientation control affects the manifestation of properties. The presentation will probe how defects induced during growth can give rise to “ferroelectric-like” function and a pathway to tunable dielectric response. In turn, we will explore how antiferroelectrics offer a pathway to overcome traditional limitations in the achievement of large electromechanical responses in thin-film materials. In particular, we will examine how unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints together with a reversible detilting of the oxygen octahedra and lattice-volume expansion in all dimensions combine in a way such that in-plane clamping further enhances the out-of-plane expansion. We will also investigate how multilayer heterostructuring can further enhance breakdown strengths and electromechanical response, thus paving the way to exceptional performance and the potential for high-performance micro-/nano-electromechanical systems. Finally, we will explore a new direction in these materials – relaxor antiferroelectrics, which offer unique properties and capabilities beyond their parent compounds. Ultimately, antiferroelectrics represent an understudied realm of ferroic materials, ripe for the application of thin-film epitaxy to accelerate our understanding and use of these materials.

The study of antiferroelectrics has become a hot topic in recent years due to their practical use as energy-storage and solid-state-cooling devices. This has pushed the study of its archetype, lead zirconate (PZO), to the extreme of reconsidering the concept of antiferroelectricity and its very nature. Moreover, unifying the microscopic picture of antiferroelectricity with its macroscopic manifestation is still a pending task.

New findings on PZO (new theoretical ground states, ferrielectricity in thin films, low temperature phase transitions...) have posed more questions than answers. The established ground state, the orthorhombic 40-atom cell with Pbam space group, has been challenged both theoretically and experimentally. The main lattice instabilities of PZO (several soft modes of different symmetries) have already been studied and it is long known that they couple to trigger the AFE state. However, intermediate polar or antipolar states compete and their presence depends on a delicate energetic balance that can be disturbed by several factors across the phase transition, potentially leading to a ferrielectric state.
In this talk I will review our recent efforts to identify the ground state of PZO at very low temperatures, including dielectric measurements and polarised Raman scattering experiments compared to DFT calculations.
The experiments used extremely chemically pure oriented single crystals, with specific cuts to measure in all possible orientations and access all phonon symmetries. The Raman results were contrasted with predictions from group analysis and DFT calculations for other reported structures (the 80-atom Pnam cell and the 30-atom ferrielectric I2cm cell) in terms of number of active modes and frequencies.
All experimental results show no traces of additional phase transitions down to 4 K, apart from the main one near 503 K, and the Raman spectra down to He temperatures could be explained by the commonly accepted Pbam antiferroelectric structure. This leaves open the question of whether pure bulk PZO is more resilient to show ferrielectricity than for instance thin films or whether we might need to use other experimental techniques to find it.

Antiferroelectrics have lately undergone an accrued interest, due to their high applicability as capacitors for energy storage devices or in power electronics. The high-temperature stability of inorganic dielectric materials is a definite asset for some applications.
In particular, lead lanthanum zirconate titanate (PLZT) is the main antiferroelectric material currently used in the industry. Low-titanium, low-lanthanum PLZT ceramics display a rich temperature phase diagram, composed of antiferroelectric and ferroelectric phases, which drastically changes depending on chemical substitution. The study of these sequences of phase transitions is hence particularly relevant to find further applications for PLZT ceramics.
In a recent study, we used Raman spectroscopy to show that the heating/cooling rates and sample history of PbZr₀.₉₅Ti₀.₀₅O₃ can induce different sequences of phase transitions, as they can help to separately stabilise lattice instabilities. We have also shown that three-state thermal switches could be designed by cleverly exploiting these extrinsic factors.
In this study, we compare the Raman spectra and calorimetry data of Pb₀.₉₈La₀.₀₂Zr₀.₉₅Ti₀.₀₅O₃ with those of previously-studied PbZr₀.₉₅Ti₀.₀₅O₃ to assess the effect of low-lanthanum substitution on the phase transition sequence and its dynamics. In particular we investigate the previously-reported commensurate antiferroelectric to incommensurate antiferroelectric phase transition, which occurs around 360 K.

We would like to acknowledge Maria Kosec for providing these ceramics more than 25 years ago. They are still proving to be of interest to us till today, or nowadays.