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The dynamics of polarization evolution and rotation in 0.68PbMg1/3Nb2/3O3-0.32PbTiO3relaxor ferroelectric thin films are studied via in operandosynchrotron-based X-ray diffraction with AC electric fields. A frequency-limited suppression of polarization rotation was observed above ultrasonic frequencies (≳20 kHz). The nature of this suppression is informed by scanning transmission electron microscopy in the zero-field state, where a high density of nanoscale, low-angle domain walls was observed. In combination with switching dynamics studies, the results suggest that the suppression of polarization rotation at ultrasonic frequencies is due to the large activation field needed to move the domain walls when the polarization rotates between different monoclinic phases. These results are critical in understanding piezoelectric relaxation phenomena in relaxor ferroelectrics.

Harvesting waste heat for useful purposes is an essential component of improving the efficiency of primary energy utilization. Today, approaches such as pyroelectric energy conversion are receiving renewed interest for their ability to turn wasted energy back into useful energy. From this perspective, the need for these approaches, the basic mechanisms and processes underlying their operation, and the material and device requirements behind pyroelectric energy conversion are reviewed, and the potential for advances in this area is also discussed. With two-thirds of the primary energy produced every year rejected as heat, the need for techniques that harvest low-grade waste heat with higher fractions of Carnot efficiency is clear. This article develops a perspective on pyroelectric energy conversion (PEC), that leverages the intrinsic coupling between electrical polarization and temperature in pyroelectric materials where a change in temperature begets a flow of electrical charge. This article will shed light on what thermo-electrical properties are crucial for PEC and the routes to enhance them. Subsequent discussion will cover thermodynamic cycles and device design rules to extract maximum work and power.

Applications such as solid-state waste-heat energy conversion, infrared sensing, and thermally-driven electron emission rely on pyroelectric materials (a subclass of dielectric piezoelectrics) which exhibit temperature-dependent changes in polarization. Although enhanced dielectric and piezoelectric responses are typically found at polarization instabilities such as temperature- and chemically induced phase boundaries, large pyroelectric effects have been primarily limited in study to temperature-induced phase boundaries. Here, we directly identify the magnitude and sign of the intrinsic, extrinsic, dielectric, and secondary pyroelectric contributions to the total pyroelectric response as a function of chemistry in thin films of the canonical ferroelectric PbZr1–xTixO3(x= 0.40, 0.48, 0.60, and 0.80) across the morphotropic phase boundary. Using phase-sensitive frequency and applied dc-bias methods, the various pyroelectric contributions were measured. It is found that the total pyroelectric response decreases systematically as one moves from higher to lower titanium contents. This arises from a combination of decreasing intrinsic response (−232 to −97 μC m–2K–1) and a sign inversion (+33 to −17 μC m–2K–1) of the extrinsic contribution upon crossing the morphotropic phase boundary. Additionally, the measured secondary and dielectric contributions span between −70 and −29 and 10–115 μC m–2K–1under applied fields, respectively, following closely trends in the piezoelectric and dielectric susceptibility. These findings and methodologies provide novel insights into the understudied realm of pyroelectric response.

Polarization switching is a fundamental feature of ferroelectric materials, enabling a plethora of applications and captivating the attention of the scientific community for over half a century. Many previous studies considered ferroelectric switching as a purely physical process, whereas polarization is fully controlled by the superposition of electric fields. However, screening charge is required for thermodynamic stability of the single domain state that is of interest in many technological applications. The screening process has always been assumed to be fast; thus, the rate-limiting phenomena were believed to be domain nucleation and domain wall dynamics. In this manuscript, we demonstrate that polarization switching under an atomic force microscopy tip leads to reversible ionic motion in the top 3 nm of PbZr0.2Ti0.8O3surface layer. This evidence points to a strong chemical component to a process believed to be purely physical and has major implications for understanding ferroelectric materials, making ferroelectric devices, and interpreting local ferroelectric switching.

Epitaxial strain has been widely used to tune crystal and domain structures in ferroelectric thin films. New avenues of strain engineering based on varying the composition at the nanometer scale have been shown to generate symmetry breaking and large strain gradients culminating in large built-in potentials. In this work, we develop routes to deterministically control these built-in potentials by exploiting the interplay between strain gradients, strain accommodation, and domain formation in compositionally graded PbZr1–xTixO3heterostructures. We demonstrate that variations in the nature of the compositional gradient and heterostructure thickness can be used to control both the crystal and domain structures and give rise to nonintuitive evolution of the built-in potential, which does not scale directly with the magnitude of the strain gradient as would be expected. Instead, large built-in potentials are observed in compositionally-graded heterostructures that contain (1) compositional gradients that traverse chemistries associated with structural phase boundaries (such as the morphotropic phase boundary) and (2) ferroelastic domain structures. In turn, the built-in potential is observed to be dependent on a combination of flexoelectric effects (i.e., polarization–strain gradient coupling), chemical-gradient effects (i.e., polarization–chemical potential gradient coupling), and local inhomogeneities (in structure or chemistry) that enhance strain (and/or chemical potential) gradients such as areas with nonlinear lattice parameter variation with chemistry or near ferroelastic domain boundaries. Regardless of origin, large built-in potentials act to suppress the dielectric permittivity, while having minimal impact on the magnitude of the polarization, which is important for the optimization of these materials for a range of nanoapplications from vibrational energy harvesting to thermal energy conversion and beyond.

Epitaxial PbHf1–xTixO3/SrTiO3(001) thin-film heterostructures are studied for a potential morphotropic phase boundary (MPB) akin to that in the PbZr1–xTixO3system. End members, PbHfO3and PbTiO3, were found to possess orthorhombic (Pbam) and tetragonal (P4mm) crystal structures and antiferroelectric and ferroelectric (∼87 μC/cm2) behavior, respectively. PbHf0.75Ti0.25O3and PbHf0.25Ti0.75O3solid solutions were both found to be ferroelectric with rhombohedral (R3c, ∼22 μC/cm2) and tetragonal (P4mm, ∼46 μC/cm2) structures, respectively. For intermediate PbHf1–xTixO3compositions (e.g., x= 0.4, 0.45, 0.5, and 0.55), a structural transition was observed from rhombohedral (hafnium-rich) to tetragonal (titanium-rich) phases. These intermediate compositions also exhibited mixed-phase structures including R3c, monoclinic (Cm), and P4mmsymmetries and, in all cases, were ferroelectric with remanent (5–22 μC/cm2) and saturation (18.5–36 μC/cm2) polarization and coercive field (24–34.5 kV/cm) values increasing with x. While the dielectric constant was the largest for PbHf0.6Ti0.4O3, the MPB is thought to be near x= 0.5 after separation of the intrinsic and extrinsic contributions to the dielectric response. Furthermore, piezoelectric displacement–voltage hysteresis loops were obtained for all chemistries revealing displacement values as good as PbZr0.52Ti0.48O3films in the same geometry. Thereby, the PbHf1–xTixO3system is a viable alternative to the PbZr1–xTixO3system offering comparable performance.