As a major branch of hybrid perovskites, two-dimensional (2D) hybrid double perovskites are expected to be ideal systems for exploring novel ferroelectric properties, because they can accommodate a variety of organic cations and allow diverse combinations of different metal elements. However, no 2D hybrid double perovskite ferroelectric has been reported since the discovery of halide double perovskites in the 1930s. Based on trivalent rare-earth ions and chiral organic cations, we have designed a new family of 2D rare-earth double perovskite ferroelectrics, A4MIMIII(NO3)8, where A is the organic cation, MI is the alkaline metal or ammonium ion, and MIII is the rare-earth ion. This is the first time that ferroelectricity is realized in 2D hybrid double perovskite systems. These ferroelectrics have achieved high-temperature ferroelectricity and photoluminescent properties. By varying the rare-earth ion, variable photoluminescent properties can be achieved. The results reveal that the 2D rare-earth double perovskite systems provide a promising platform for achieving multifunctional ferroelectricity.

Piezoelectric materials are technologically important, and the most used are perovskite ferroelectrics. In recent years, more and more emerging areas have put forward new requirements for piezoelectric materials, such as light weight, low acoustic impedance, good flexibility, and biocompatibility. In this context, hybrid organic–inorganic perovskite ferroelectrics have emerged as promising supplements, because they combine attractive features of inorganic and organic materials. Among them, hybrid double-metal perovskites have recently been found to exhibit excellent ferroelectricity. However, their potential as piezoelectric materials has not been exploited. Here, we describe large piezoelectric response in hybrid rare-earth double perovskite relaxor ferroelectrics (RM3HQ)2­RbLa­(NO3)6 and (RM3HQ)2­NH4La­(NO3)6 (RM3HQ = R-N-methyl-3-hydroxyl­quinuclidinium). They are simultaneously ferroelectric and ferroelastic crystals, with the R3 ferroelectric phase and P213 paraelectric phase. We found that ferroelectric polar microdomains and paraelectric nonpolar regions coexist in a wide temperature range through variable-temperature piezoresponse force microscopy images. The two-phase coexistence reveals low energy barriers of transitions between the two phases and between the polar microdomains with different polarization directions. These lead to the easy polarization rotation of the polar microdomains upon applying a stress and, accordingly, the large piezoelectric response up to 106 pC N–1 for (RM3HQ)2­RbLa­(NO3)6. This finding represents a significant step toward novel applications of piezoelectric materials based on lead-free hybrid perovskites.

Owing to the merits of giant power density and ultrafast charge–discharge time, dielectric capacitors including ceramics and films have inspired increasing interest lately. Nevertheless, the energy storage density of dielectric ceramics should be further optimized to cater to the boosting demand for the compact and portable electronic devices. Herein, an ultrahigh recoverable energy storage density W rec of 13.44 J/cm3 and a high efficiency η of 90.14% are simultaneously realized in BiFeO3–BaTiO3–NaTaO3 relaxor ferroelectric ceramics with high polarization P max, reduced remanent polarization P r, and optimized electric breakdown strength E b. High P max originates from the genes of BiFeO3-based ceramics, and reduced P r is induced by enhanced relaxor behavior. Particularly, a large E b is achieved by the synergic contributions from complicated internal and external factors, such as decreased grain size and improved resistivity and electrical homogeneity. Furthermore, the ceramics also exhibit satisfactory frequency, cycling and thermal reliability, and decent charge–discharge property. This work not only indicates that the BiFeO3-based relaxor ferroelectric materials are promising choices for the next-generation electrostatic capacitors but also paves a potential approach to exploit novel high-performance dielectric ceramics.

Lead-free antiferroelectric ceramics have drawn widespread interest recently on account of their environmentally friendly components and potential applications in high-power systems. However, their relatively low recoverable energy storage density (W rec < 10 J/cm 3 ), limited by the electric breakdown strength (E b < 60 kV/mm), and low efficiency (η < 80%), generated by large hysteresis during the antiferroelectric−ferroelectric phase transition, have seriously restricted their application in portable and compact electronic devices. In this study, the relaxor antiferroelectric (1− x)NaNbO 3−x (0.55BiFeO 3 -0.45SrTiO 3 ) ceramics were elaborately designed and systematically explored. With the help of composition regulation, the ceramics not only exhibited a stable antiferroelectric phase but also underwent a structural transformation from an antiferroelectric P (Pbma) phase to R (Pnma) phase, as confirmed by the temperature-dependent dielectric constants, Raman spectra, X-ray diffraction (XRD) refinement, pinched polarization-electric field (P−E) curves, and four-peak current-electric field (I−E) curves. In addition, the relaxor characteristic was demonstrated by the diffuse dielectric peaks, slim P−E curves, flattened Raman peaks, and I−E curves. Consequently, novel relaxor antiferroelectric ceramics were successfully obtained, which simultaneously revealed the features of relaxor and antiferroelectricity. Specifically, the E b was significantly improved because of the reduced grain size, small sample thickness, exceptionally low dielectric loss, and a moderate dielectric constant. Finally, the sample with x = 0.12 showed an ultrahigh W rec value of 16.2 J/cm 3 and satisfactory η of 82.3% at 97 kV/mm, outperforming most state-of-the-art counterparts. Furthermore, the ceramic also exhibited a large current density (C D ) of 1268.4 A/cm 2 and power density (P D ) of 177.6 MW/cm 3 at 28 kV/mm, offering prospective applications in high-power capacitors. This work not only achieved outstanding comprehensive energy storage performance in sodium niobate-based ceramics by modulating the antiferroelectric structure but also provided a feasible route to developing high-performance dielectric capacitors from the viewpoint of structure−property relationship.

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