Chemical design of lead-free relaxors with simultaneously high energy density (W rec) and high efficiency (η) for capacitive energy-storage has been a big challenge for advanced electronic systems. The current situation indicates that realizing such superior energy-storage properties requires highly complex chemical components. Herein, we demonstrate that, via local structure design, an ultrahigh W rec of 10.1 J/cm3, concurrent with a high η of 90%, as well as excellent thermal and frequency stabilities can be achieved in a relaxor with a very simple chemical composition. By introducing 6s 2 lone pair stereochemical active Bi into the classical BaTiO3 ferroelectric to generate a mismatch between A- and B-site polar displacements, a relaxor state with strong local polar fluctuations can be formed. Through advanced atomic-resolution displacement mapping and 3D reconstructing the nanoscale structure from neutron/X-ray total scattering, it is revealed that the localized Bi enhances the polar length largely at several perovskite unit cells and disrupts the long-range coherent Ti polar displacements, resulting in a slush-like structure with extremely small size polar clusters and strong local polar fluctuations. This favorable relaxor state exhibits substantially enhanced polarization, and minimized hysteresis at a high breakdown strength. This work offers a feasible avenue to chemically design new relaxors with a simple composition for high-performance capacitive energy-storage.

Relaxor ferroelectrics are known for outstanding piezoelectric properties, finding a broad range of applications in advanced electromechanical devices. Decoding the origins of the enhanced properties, however, have long been complicated by the heterogeneous local structures. Here, we employ the advanced big-box refinement method by fitting neutron-, X-ray-based total scattering, and X-ray absorption spectrum simultaneously, to extract local atomic polar displacements and construct 3D polar configurations in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3–PbTiO3. Our results demonstrate that prevailing order-disorder character accompanied by the continuous rotation of local polar displacements commands the composition-driven global structure evolution. The omnidirectional local polar disordering appears as an indication of macroscopic relaxor characteristics. Combined with phase-field simulations, it demonstrates that the competing local polar order-disorder between different states with balanced local polar length and direction randomness leads to a flattening free-energy profile over a wide polar length, thus giving rise to high piezoelectricity. Our work clarifies that the critical structural feature required for high piezoelectricity is the competition states of local polar rather than relaxor.

Piezoelectric ceramics have been extensively used in actuators, where the magnitude of electrostrain is key indicator for large-stroke actuation applications. Here, we propose an innovative strategy based on defect chemistry to form a defect-engineered morphotropic phase boundary and achieve a giant strain of 1.12% in lead-free Bi 0.5 Na 0.5 TiO 3 (BNT)–based ceramics. The incorporation of the hypothetical perovskite BaAlO 2.5 with nominal oxygen defect into BNT will form strongly polarized directional defect dipoles, leading to a strong pinning effect after aging. The large asymmetrical strain is mainly attributed to two factors: The defect dipoles along crystallographic [001] direction destroy the long-range ordering of the ferroelectric and activate a reversible phase transition while promoting polarization rotation when the dipoles are aligned along the applied electric field. Our results not only demonstrate the potential application of BNT-based materials in low-frequency, large-stroke actuators but also provide a general methodology to achieve large strain.

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A single tetragonal phase with a tiny axial ratio and large oxygen octahedron tilt exhibits boosted piezoelectricity with excellent thermal stability.

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Lead-based relaxor ferroelectrics feature exceptional dielectric and electromechanical properties. The inherent local structural inhomogeneity remains poorly characterized and impedes the understanding of their unique electrical behavior. Herein, the local structure of a technologically important 0.75Pb(Mg 1/3 Nb 2/3 )O 3 −0.25PbTiO 3 relaxor is studied as a function of temperature, by employing neutron total scattering combined with newly advanced reverse Monte Carlo modeling. The statistical analysis of local cationic polar displacement vectors demonstrates that a prevailing order/disorder feature and strong displacive behavior are observed in Pb and Ti, but both weak components are observed in Nb/Mg. This leads to trifurcated polar behavior across the feature temperatures. Importantly, these distinct cationic polar vectors are correlated and present a microscopic picture, where sporadic correlated orthorhombic polar clusters are embedded in a long-range rhombohedral polar matrix at low temperature. Their fraction extends with increasing temperature, presenting strong ferroelectricity with dielectric relaxation macroscopically. Furthermore, the long-range rhombohedral polar ordering is disrupted by temperature, forming free-oriented polar clusters with reducing electric dipole length, gradually losing their macroscopic polarization, and resulting in a diffuse phase transition behavior. Altogether, this offers a fundamental basis in terms of local polar fluctuations in inferring the nature of the unique properties of relaxor ferroelectrics.

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