Lead zinc niobate-lead titanate (PZN-PT) system is of particular interest for scientific researches and commercial applications due to the unique relaxation feature and superior electromechanical responses. However, it is difficult to prepare polycrystalline ceramics near the morphotropic phase boundary due to the stability of a competing lead niobate pyrochlore phase. BaTiO3 (BT) was reported to be an effective perovskite phase stabilizer at a cost of reduction in Curie temperature and piezoelectric properties. Herein, the amount of BT in PZN-PT-BT system and the sintering conditions were optimized to simultaneously stabilize the perovskite phase and maintain high dielectric and piezoelectric response. An optimum piezoelectric coefficient d(33) similar to 870 pC/N along with a T-m similar to 133 degrees C and an electromechanical coupling coefficient k(33) similar to 0.61 was obtained in the PZN-8PT-6BT ceramics sintered at 1100 degrees C. The low sintering temperature, wide processing window, and improved dielectric and piezoelectric properties make PZN-PT-BT ceramics potential candidates for piezoelectric devices.

A two-dimensional (2-D) array with a small pitch (approximately $0.5\lambda $ in medium) can achieve complete 3-D control of ultrasound beams without grating lobes and enable the generation of multiple focal spots simultaneously, which is a desired tool for noninvasive therapy. However, the large electrical impedance of 2-D array elements owing to their small size results in a low energy transfer efficiency between a 2-D array and an electrical system, thereby limiting their practical applications. This article presents the development of a 1-MHz 256-element 2-D array ultrasonic transducer of low electrical impedance based on a new Sm-doped Pb(Mg1/3Nb2/3)O3–PbTiO3 (Sm-PMN-PT) piezoceramic with ultrahigh dielectric permittivity. The electrical impedance of the array element is decreased by 3.4 times as the Sm-PMN-PT replacing commercial PZT-5H. Consequently, the output acoustic pressure of the 2-D array made of Sm-PMN-PT ceramic is approximately twice that of the 2-D array made of PZT-5H ceramic under the same excitation conditions. Array elements are spaced at a 1.1-mm pitch ( $0.71\lambda $ in water), enabling a large steering range of the ultrasound beam. A multiple-target blood–brain barrier opening in vivo is demonstrated using the proposed 2-D array with electronic focusing and steering. The obtained results indicate that the 2-D array made of Sm-PMN-PT ceramic is promising for practical use in low-intensity ultrasound therapy applications.

Designing high-performance piezoelectric materials based on atomic-scale calculations is highly desired in recent years, following the understanding of the structure-property relationship of state-of-the-art piezoelectric materials. Previous mesoscale simulations showed that local structural heterogeneity plays an important role in the piezoelectric property of ferroelectrics; that is, larger structural heterogeneity leads to higher piezoelectricity. In this Rapid Communication, by combining first-principles calculations and experimental characterizations, we explored the atomic-scale origin of the high piezoelectricity for samarium-doped Pb(Mg1/3Nb2/3)O-3-PbTiO3 (PMN-PT) ceramics, which possesses the highest piezoelectric d(33) of similar to 1500 pC N-1 among all known piezoelectric ceramics. The impacts of various dopants on local structure and piezoelectric properties of PMN-PT ceramics were investigated in terms of the effective ionic radius and cation valence. Our results show that A-site dopants with a valence of 3+ are more effective to produce local structural heterogeneity in PMN-PT when compared with the A-site dopants with a valence of 2+, and a smaller dopant size leads to a larger variation of local structure. According to this study, the outstanding piezoelectricity in Sm-doped PMN-PT ceramics is attributed to the fact that Sm3+ is the smallest ions that can entirely go to the A site of PMN-PT rather than the B site. The present work may benefit the design of high-performance piezoelectric materials based on the concept of local structural engineering.

High-performance piezoelectrics benefit transducers and sensors in a variety of electromechanical applications. The materials with the highest piezoelectric charge coefficients (d 33) are relaxor-PbTiO3 crystals, which were discovered two decades ago. We successfully grew Sm-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Sm-PMN-PT) single crystals with even higher d 33 values ranging from 3400 to 4100 picocoulombs per newton, with variation below 20% over the as-grown crystal boule, exhibiting good property uniformity. We characterized the Sm-PMN-PT on the atomic scale with scanning transmission electron microscopy and made first-principles calculations to determine that the giant piezoelectric properties arise from the enhanced local structural heterogeneity introduced by Sm3+ dopants. Rare-earth doping is thus identified as a general strategy for introducing local structural heterogeneity in order to enhance the piezoelectricity of relaxor ferroelectric crystals.

Piezoelectric materials, which respond mechanically to applied electric field and vice versa, are essential for electromechanical transducers. Previous theoretical analyses have shown that high piezoelectricity in perovskite oxides is associated with a flat thermodynamic energy landscape connecting two or more ferroelectric phases. Here, guided by phenomenological theories and phase-field simulations, we propose an alternative design strategy to commonly used morphotropic phase boundaries to further flatten the energy landscape, by judiciously introducing local structural heterogeneity to manipulate interfacial energies (that is, extra interaction energies, such as electrostatic and elastic energies associated with the interfaces). To validate this, we synthesize rare-earth-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT), as rare-earth dopants tend to change the local structure of Pb-based perovskite ferroelectrics. We achieve ultrahigh piezoelectric coefficients d33 of up to 1,500 pC N-1 and dielectric permittivity ε33/ε0 above 13,000 in a Sm-doped PMN-PT ceramic with a Curie temperature of 89 °C. Our research provides a new paradigm for designing material properties through engineering local structural heterogeneity, expected to benefit a wide range of functional materials.

Relaxor ferroelectrics are a type of ferroelectrics whose long-range ordering of dipoles is disrupted by cation disorder, resulting in complex polar states at the nanoscale. Based on the understanding of relaxor ferroelectrics, in article number 1801504, Fei Li and co-workers discuss potential strategies to exploit the local structural heterogeneity to design ferroelectrics for drastically enhancing their electromechanical performances.

Piezoelectric materials are essential parts of the electronics and electrical equipment used for consumer and industrial applications, such as ultrasonic piezoelectric transducers, sensors, actuators, transformers, and resonators. In this review, the development of piezoelectric materials and the figures of merit for various electromechanical applications are surveyed, focusing on piezoelectric crystals, i.e., the high-performance relaxor-PbTiO3-based perovskite ferroelectric crystals and nonferroelectric high-temperature piezoelectric crystals. The uniqueness of these crystals is discussed with respect to different usages. Finally, the existing challenges and perspective for the piezoelectric crystals are discussed, with an emphasis on the temperature-dependent properties, from cryogenic temperatures up to the ultrahigh-temperature usage range.

Long-range ordering of dipoles is a key microscopic signature of ferroelectrics. These ordered dipoles form ferroelectric domains, which can be reoriented by electric fields. Relaxor ferroelectrics are a type of ferroelectric where the long-range ordering of dipoles is disrupted by cation disorder, exhibiting complex polar states with a significant amount of local structural heterogeneity at the nanoscale. They are the materials of choice for numerous devices such as capacitors, nonlinear optical devices, and piezoelectric transducers, owing to their extraordinary dielectric, electro-optic, and electromechanical properties. However, despite their extensive applications in these devices, the origins of their unique properties are yet to be fully understood, hindering the design and exploration of new relaxor ferroelectric-based materials. Herein, the complex polar states and applications of relaxor ferroelectrics are first introduced. Attention is then focused on their electromechanical properties, where the relationship between local structural heterogeneity and the extraordinary electromechanical properties is discussed. Based on the understanding of relaxor ferroelectrics, potential strategies to exploit the local structural heterogeneity to design ferroelectrics for drastically enhancing their electromechanical performances are also discussed. It is expected that this article will stimulate future studies on the important roles of local structural heterogeneity in improving the properties of various functional materials.

A modified Pb(Mg1/3Nb2/3)O-3-PbTiO3 (PMN-PT) polycrystalline ceramic with ultrahigh relative clamped dielectric permittivity (epsilon(S)/epsilon(0) = 3500) and high piezoelectric properties (d(33) = 1200 pC/N, k(t) = 0.55) was used to fabricate high-frequency miniature ultrasound transducers. A 39-MHz high-frequency ultrasound needle transducer with a miniature aperture of 0.4 mm x 0.4 mm was designed and successfully characterized. The fabricated needle transducer had an electromechanical coupling factor k(t) of 0.55, large bandwidth of 80% at -6 dB, and low insertion loss of -13 dB. A wire phantom and porcine eyeball imaging study showed good imaging capability of this needle transducer. The transducer performance was found to be superior to that of other needle transducers with miniature apertures, making this modified PMN-PT ceramic-based needle transducer quite promising for minimally invasive procedures in medical applications.

Four series of Pb(Mg1/3Nb2/3)O3-Pb(In1/2Nb1/2)O3-PbZrO3-PbTiO3 (PMN-PIN-PZ-PT) quaternary ceramics with compositions located at the morphotropic phase boundary (MPB) regions were prepared. The MPBs of the multicomponent system were predicted using a linear combination rule and experimentally confirmed by X-ray powder diffraction and electrical measurement. The positions of MPBs in multicomponent systems were found in linear correlation with the tolerance factor and ionic radii of non-PT end-members. The phase structure, piezoelectric coefficient, electromechanical coupling coefficient, unipolar strains, and dielectric properties of as-prepared ceramics were systematically investigated. The largest d33s were obtained at S36.8, L37.4, M39.6, and N35.8, with the corresponding values of 580, 450, 420, and 530 pC/N, respectively, while the largest kps were found at S34.8, L37.4, M39.6, and N35.8, with the respective values of 0.54, 0.50, 0.47, and 0.53. The largest unipolar strain Smax and high-field piezoelectric strain coefficients d33* were also observed around the respective MPB regions. The rhombohedral-to-tetragonal phase transition temperature Trt increased with increasing PIN and PZ contents. Of particular importance is that high Trt of 140-197 °C was achieved in the M series with PZ and PIN contents being around 0.208 and 0.158, which will broaden the temperature usage range.

The pyroelectric properties of Nb(Mn)‐doped and Nb/Mn co‐doped CaBi4Ti4O15 (CBT) bismuth layer‐structured ferroelectric ceramics were investigated. It was found that Nb/Mn co‐doping resulted in stronger enhancement of pyroelectric properties than that of single Nb or Mn doping. The mechanism of doping effect was explained by the distortion of the [BO6] octahedra induced by the doped Nb and Mn cations occupying the B‐site of the pseudoperovskite structure. A large pyroelectric coefficient of 84.4 μC/m2K was obtained at room temperature for Nb/Mn co‐doped CBT (CBTN‐Mn) ceramics, higher than that of pure, Nb or Mn‐doped counterparts, being on the order of 35.9, 58.2, 44.0 μC/m2K, respectively. The enhanced pyroelectric coefficient together with reduced dielectric constant (99) and dielectric loss (0.002) led to greater improvement of figures of merit (FOMs), including FOMs for voltage responsivity (Fv ~ 3.95 × 10−2 m2/C) and detectivity (Fd ~ 2.44 × 10−5 Pa−1/2), in CBTN‐Mn ceramics. Furthermore, the temperature variations of Fv and Fd were found to be 24% and 68%, respectively, over a broad temperature range from room temperature to 350°C, making CBTN‐Mn ceramics potential candidate for high‐temperature pyroelectric devices.

The discovery of ultrahigh piezoelectricity in relaxor-ferroelectric solid solution single crystals is a breakthrough in ferroelectric materials. A key signature of relaxor-ferroelectric solid solutions is the existence of polar nanoregions, a nanoscale inhomogeneity, that coexist with normal ferroelectric domains. Despite two decades of extensive studies, the contribution of polar nanoregions to the underlying piezoelectric properties of relaxor ferroelectrics has yet to be established. Here we quantitatively characterize the contribution of polar nanoregions to the dielectric/piezoelectric responses of relaxor-ferroelectric crystals using a combination of cryogenic experiments and phase-field simulations. The contribution of polar nanoregions to the room-temperature dielectric and piezoelectric properties is in the range of 50-80%. A mesoscale mechanism is proposed to reveal the origin of the high piezoelectricity in relaxor ferroelectrics, where the polar nanoregions aligned in a ferroelectric matrix can facilitate polarization rotation. This mechanism emphasizes the critical role of local structure on the macroscopic properties of ferroelectric materials.

The electromechanical properties of [111]-oriented tetragonal Pb(In1/2Nb1/2O3)-Pb(Mg1/3Nb2/3O3)-PbTiO3 (PIN-PMN-PT) crystals were investigated for potential high frequency ultrasonic transducers. The domain-engineered tetragonal crystals exhibit an ultrahigh free dielectric permittivity ε33T > 10 000 with a moderate electromechanical coupling factor k33 ∼ 0.79, leading to a high clamped dielectric permittivity ε33S of 2800, significantly higher than those of the rhombohedral relaxor-PT crystals and high-K (dielectric permittivity) piezoelectric ceramics. Of particular significance is that the [111]-oriented tetragonal crystals were found to possess high elastic stiffness, with frequency constant N33 of ∼2400 Hz m, allowing relatively easy fabrication of high-frequency transducers. In addition, no scaling effect of piezoelectric and dielectric properties was observed down to thickness of 0.1 mm, corresponding to an operational frequency of ∼24 MHz. These advantages of [111]-oriented tetragonal PIN-PMN-PT crystals will benefit high-frequency ultrasonic array transducers, allowing for high sensitivity, broad bandwidth, and reduced noise/crosstalk.

Unique properties have been obtained by defect dipole engineering in relaxor-PT single crystals.

1- x - y)BiFeO3-x(K0.5Bi0.5)TiO3-yPbTiO3 (BFKBT- PT) piezoelectric ceramics were investigated across the compositional space and contrasted against the xBiFeO3- (1-x)(K0.5Bi0.5)TiO3 (BF-KBT) system, whereby a range of relaxor-like/ferroelectric behavior was observed. Structural and piezoelectric properties were closely related to the PbTiO3 concentration; below a critical concentration, relaxor-like behavior was identified. The mechanisms governing the piezoelectric behavior were investigated with structural, electrical, and imaging techniques. X-ray diffraction established that longrange non-centrosymmetric crystallographic order was evident above a critical PbTiO3 concentration, y > 0.1125. Commensurate with the structural analysis, electric-field-induced strain responses showed electrostrictive behavior in the PbTiO3-reduced compositions, with increased piezoelectric switching in PbTiO3-rich compositions. Positive-up-negative-down (PUND) analysis was used to confirm electric-field-induced polarization measurements, elucidating that the addition of PbTiO3 increased the switchable polarization and ferroelectric ordering. Piezoresponse force microscopy (PFM) of the BF-KBT-PT system exhibited typical domain patterns above a critical PbTiO3 threshold, with no ferroelectric domains observed in the BF-KBT system in the pseudocubic region. Doping of BiFeO3-PbTiO3 has been unsuccessful in the search for hightemperature materials that offer satisfactory piezoelectric properties; however, this system demonstrates that the partial substitution of alternative end-members can be an effective method. The partial substitution of PbTiO3 into BF-KBT enables long-range non-centrosymmetric crystallographic order, resulting in increased polar order and TC, compared with the pseudocubic region. The search for novel high-temperature piezoelectric ceramics can therefore exploit the accommodating nature of the perovskite family, which allows significant variance in chemical and physical characters in the exploration of new solid-solutions.

Dielectric permittivity of [111]-oriented Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) single crystals with various PT content and phase were investigated as a function of temperature and applied DC electric field. The temperature dependent dielectric permittivity with different PT content showed various dielectric anomalies, revealing different ferroelectric phase at room temperature. The dielectric permittivity-electric field (ε-E) measurement showed that the dielectric permittivity was strongly dependent on the applied electric field and phase structures, where different dielectric anomalies can be observed as a function of DC bias and phase, being inherently associated with the electric field induced domain switching and phase transition.