Unprecedented Electromechanical Response in Antiferroelectric Thin Films

Antiferroelectrics, long considered the somewhat less exciting cousin of ferroelectrics, are having their moment. These materials, which possess antipolar order (<i>i.e.</i>, 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. While these materials have been known for many decades, they remain relatively less well studied and understood as compared to their ferroelectric relatives. To better understand the nature of such phase transitions in antiferroelectrics and to finely engineer the polarization properties for targeted applications requires that one can fabricate high-quality versions of antiferroelectric materials. In this spirit, recent years have seen a growth in efforts to study these materials, particularly as thin films. Epitaxial films offer researchers the opportunity to finely control and manipulate the structure, orientation, strain, and much more. In turn, we are offered unprecedented insights into the nature and evolution of these complex and, at times, poorly understood physical phenomena.<br/> <br/>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. Our attention will focus on classic, prototypical antiferroelectric materials such as PbZrO₃ and PbHfO₃ and solid solutions and multilayers derived from these parent materials. In turn, we will 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. In particular, we will explore how antiferroelectrics offer a pathway to overcome traditional limitations in the achievement of large electromechanical responses in thin-film materials. We will explore an unconventional coupling of the field-induced antiferroelectric-to-ferroelectric phase transition and the substrate constraints in these materials. A detilting of the oxygen octahedra and lattice-volume expansion in all dimensions are observed commensurate with the phase transition, such that the in-plane clamping further enhances the out-of-plane expansion. In turn, an abnormal thickness scaling is realized wherein an ultrahigh electromechanical strain (1.7%) is produced from a model antiferroelectric PbZrO₃ film of just 100 nm thick. The exceptional performance and novel mechanism provide a promising pathway to develop high-performance micro-/nano-electromechanical systems. Ultimately, antiferroelectrics represent a relatively understudied realm of ferroic materials, ripe for the application of thin-film epitaxy to accelerate our understanding and use of these interesting materials.