Antiferroelectric PbZrO₃ and PbHfO₃ Thin Films—Anisotropic Control of Functionalities and Phase Transitions

Antiferroelectrics (AFE) are materials with an antipolar arrangement of dipoles in adjacent unit cells, resulting in an overall, macroscopic null net polarization in absence of applied external fields. However, applying a sufficiently large electric field induces a transition from antipolar to polar, through parallel alignment of the dipoles. At reduction of electric field, the material returns to an antipolar phase in a nonlinear and hysteretic fashion, consistent with a ferroelectric material. The antipolar to polar phase transition (and <i>vice versa</i>) is accompanied in perovskite AFE oxides with development of very large strains, extreme changes in dielectric permittivity, and often negative electrocaloric effects. Hence, these materials are of interest for applications in large force-large displacement actuators, multi-state memories, high power capacitors, negative tunable dielectrics, electrooptic switches, solid state cooling devices, etc.<br/>For many of these applications it is of paramount importance to control the phase transition fields, in order to control the functionalities. Such tuning of the properties has been often achieved by use of precise amounts of A-site and B-site cation doping of peroskite prototypical AFE oxides. However, such doping schemes result in challenging processing of the material, particularly in thin film form for miniaturized applications.<br/>Here we report on facile chemical solution processed lead zirconate, PbZrO₃ (PZO), and lead hafnate, PbHfO₃ (PHO), perovskite antiferroelectric thin films, deposited on platinized silicon substrates. We show that for both compositions, the transitions fields of the films (antipolar to polar and vice versa) can be modified by close to 2x, through simple selection of preferential growth direction, specifically, leveraging homogenous seed layers of PbO. Films with PbO seed layers resulted in preferential 001-orientation, with Lotgering factors above 90% and those direct deposited on platinized Si substrates showed up to 100% 042 orientation. Although no secondary phase was observed in the x-ray diffraction of the samples, the electron microscopy analysis of the cross-sections revealed presence of residual porosity and possible secondary phases at crystallization interfaces. PHO films’ forward transition films were changed in a range from ~250 to ~320 kV/cm through control of the crystallographic orientation, while the same for PZO ranged from ~300 kV/cm to ~650 kV/cm for films of approximately 200 nm in thickness. The 001-oriented films showed a higher energy storage density, and efficiency (16 J/cm² and ∼75%, respectively) for applied electric fields of ∼900 kV/cm) than 042-oriented ones (energy storage density of ∼8 J/cm² and efficiency of ∼70% at electric field as low as 400 kV/cm). Films of mixed orientation showed intermediate energy storage density but reduced efficiencies due to multiple phase transitions due to grains of varying orientation. 001-oriented PHO films showed similar energy storage efficiencies to PZO with values of up to 75%. We will furthremore discuss theoretical basis for such anisotropic behavior as well as the anisotropic electromechanical response (with strains up to 1.2%), dielectric tunability, and approaches to reduce defects.