The tri-partite project involving teams from Georgia Institute of Technology (USA), University of south Florida (USA), Trinity college Dublin (Republic of Ireland) and Queen’s University Belfast aimed to rediscover the underpinning science of a classic antiferroelectric material whose unique properties are useful in plethora of practical devices. The conventional theory of antiferroelectricity, developed by Kittel in 1950^s, describes antiferroelectric materials through an antiparallel arrangement of dipoles in adjacent unit cells at ground state, which switch to parallel arrangement at high applied electric fields. Such behavior has been historically associated with the perovskite structure PbZrO₃, the archetypal antiferroelectric and has profound implications towards multitude of applications in devices based on them. However, recent work on PbZrO₃ suggests a more complex picture, with a range of ferroelectric, ferrielectric and modulated polarization behavior at the nanoscale and debate about the nature of anti-ferroelectricity in this material has intensified.

In this context, the team has developed a bottoms-up approach in the ultrathin limit considering few atomic layers which could provide insight into the mechanism of stabilization of the polar phases over the antipolar phase seen in bulk PbZrO₃. Dr Kumar, lead PI in QUB and Head of Centre for Quantum Materials Technologies said about the findings: ‘Our experimental team has worked closely with theorists to employ first-principles density functional theory to predict the stability of polar phases in Pt/PbZrO₃/Pt nanocapacitors. In a few atomic layer thick slabs of PbZrO₃ sandwiched between Pt electrodes, we find that the polar phase originating from the well-established R3c phase of bulk PbZrO₃ is energetically favorable over the antipolar phase originating from the Pbam phase of bulk PbZrO₃. The famous triple-well potential of antiferroelectric PbZrO₃ is modified in the nanocapacitor limit in such a way as to swap the positions of the global and local minima, stabilizing the polar phase relative to the antipolar one. The size effect is decomposed into the contributions from dimensionality reduction, surface charge screening, and interfacial relaxation, which reveals that it is the creation of well-compensated interfaces that stabilizes the polar phases over the antipolar ones in nanoscale PbZrO₃. We believe that our findings shed light onto size effects in this prototypical antiferroelectric and will be valuable in interpreting diverse experimental data. The results could also stimulate non-traditional ways to achieve ferroelectricity at the extreme thickness limits.’

The work has been published recently in the high-visibility journal NPJ Computational Materials