Large reversible shear strain has been achieved by electric-field-driven bipolar switching in a hybrid ferroelectric, facilitating development of shape-memory-type actuators with outstanding figures of merit.
The ability to interconvert electrical and mechanical energy makes piezoelectric crystals an important material for consumer electronics, medical devices, scientific tools for ultra-sensitive metrology, and environmental monitoring. Current commercial applications are dominated by perovskite ceramics such as PZT (lead zirconium titanate, Pb(Zr,Ti)O3), but piezoelectric materials come in many forms including polymer, crystalline, inorganic and biological (Fig. 1). In principle, any structure that lacks a centre of symmetry can produce electricity due to mechanical stress (or conversely, deform when voltage is applied) and recently a number of two-dimensional and nanostructured materials have demonstrated significant piezoelectricity. A subset of piezoelectric materials also exhibits ferroelectricity, meaning they have the ability to switch their polarization state under an applied electric field. In addition to their electromechanical properties such as stiffness, permittivity and electromechanical coupling, for many applications the key figure of merit is the piezoelectric strain constant d. Yet, despite deepening molecular-level understanding of piezoelectric crystals, it remains a daunting task to ‘dial in’ key figures of merit. Rational design of materials with technologically useful electromechanical properties currently requires a combination of traditional trial-and-error materials synthesis, predictive modelling and characterization. In the search for lead-free piezoelectrics1, as well as piezoelectrics for novel applications2, the ability to engineer desired piezoelectricity at the atomic scale remains elusive. As the piezoelectric response of a material is a ratio of polarization to stiffness, a large piezoelectric response would arise from a material with a high remnant polarization and low stiffness. However, it is often found that soft organic materials have low polarization, and hard inorganic materials have a high polarization, which has led to the recent focus on hybrid organic–inorganic piezoelectrics.
Towards this goal, Junling Wang and colleagues3 developed an approach, published in Nature Materials, to systematically tune the electromechanical properties of the hybrid, bromine-doped ferroelectric (PTMA)CdBr3xCl3(1–x), produced from the commonly used organic polymer PTMA and the inorganic salt cadmium chloride (CdCl2). The authors demonstrate a switchable stress–strain hysteresis that occurs due to structural confinement in their crystal, creating a large piezoelectric response. This demonstration of controlled ferroelastic switching showcases an intriguing new functionality in a single crystal piezoelectric. The authors show how crystals in different ferroelastic states exhibit a macroscopic shear strain that correlates with unit cell distortion, which allowed this deceptively simple material to demonstrate substantial shear strain in excess of 20% for the Br-free (PTMA)CdCl3. This strain is two orders of magnitude higher than industry-standard PZT piezoelectric ceramic and larger than all reported shape-memory alloys. The authors engineered a very large shear piezoelectric response in the 90% Br-content (PTMA)CdBr2.7Cl0.3, measuring in E-fields of 1–10 kV cm–1 a piezoelectric coefficient d35 of up to 4,800 pm V–1, far in excess of perovskites4,5 and surpassing other categories of piezoelectrics6 (Fig. 1).
Wang and colleagues demonstrate the power of rational molecular-scale materials design for the identification and optimization of electromechanical properties. The structure of the hybrid ferroelectric suppresses polarization reversal, as it constrains flipping of the large organic moiety. By contrast, polarization switching in oxide ferroelectrics requires only a small displacement of the ions. The structural confinement of the bulky cationic organic chains in the hybrid ferroelectric material creates the high measured shear strain (Fig. 2). Using a combination of density functional theory (DFT) models and experiments, the authors show that the maximum strain achievable in the hybrid bromine-doped ferroelectric (PTMA)CdBr3xCl3(1–x) decreases with increasing Br content, as the Br-rich sample has a smaller monoclinic tilt angle than the Cl-rich sample. By comparing the maximum measured strains with the monoclinic angles in the crystals, the authors found matching shear and unit cell distortion angles for the Br-free sample but not for the 90%-Br sample. This suggests that the pure Cl sample was poled into an almost single-domain state, while the Br-rich sample was not, as substantiated by DFT calculations. Mapping of the minimum energy paths for switching between the two ferroelastic states revealed a shallower double-well landscape in the Br-rich sample, allowing easier switching from one domain to another.
While the large macroscopic size of the crystals is challenging for further characterization via piezoresponse force microscopy7, and the large number of atoms in the unit cell makes quantitative piezoelectric DFT calculations prohibitively expensive8, the authors determined the electromechanical properties using direct strain–electric field measurements. With appropriate calibration, this provided a straightforward method of characterization. The real-time videos of the bipolar ferroelastic switching provide unambiguous evidence for the large strain response. The reported 4,800 pm V–1 is the maximum ‘large-signal’ piezoelectric response obtained under high field and low frequency. For linear actuator applications in portable high-energy-density devices, further chemical engineering will be required in order to flatten the energy landscape and reduce the observed hysteresis and nonlinearity. These future steps would help ensure the reliability of the materials response in, for example, ultra-precision nanoscale positioning systems for smart manufacturing.
There remain a number of challenges for the practical implementation of this material, in particular the toxicity of the Cd-containing compound. These concerns are common to many materials candidates including Pb-based relaxor ferroelectrics used as high-efficiency energy storage and conversion materials due to their high dielectric constants, and hybrid perovskite solar cells. Following the clever exploitation by Wang and co-workers of the design principle of bond softening via chemical engineering, it should prove possible to encode novel functionality and superior performance in non-toxic, environmentally friendly compounds via further computation-guided selection across the broad palette of organic and inorganic building blocks. The device-materials community as a whole can build on this work by incorporating rational, systematic doping into their piezoelectric and ferroelectric materials, and by developing ways to introduce molecular confinement into their systems. Another hidden gem in this work is the DFT analysis, which provides a solid theoretical methodology to characterize the ground-state properties of non-centrosymmetric crystals with large unit cells, which can be used to screen for further new materials with novel properties such as reported here.
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The authors declare no competing interests.