HfO₂-based ferroelectric films have been extensively explored and utilized in the field of non-volatile memory and electrical programmability. However, the trade-off between ferroelectric polarization and dielectric constant in HfO₂ has limited the overall performance improvement of devices in practical applications. Herein, a novel approach is proposed for the Hf_0.5Zr_0.5O₂/ZrO₂ (HZO/ZrO₂) nanobilayer engineering, which can effectively regulate the phase structure evolution of HfO₂ films to construct a suitable morphotropic phase boundary (MPB). The findings highlight that the top ZrO₂ layer can regularly promote the formation of either the ferroelectric o-phase or the antiferroelectric t-phase. An ideal MPB is successfully established in HZO/ZrO₂ (6/9 nm) nanobilayer film by carefully optimizing the HZO/ZrO₂ thickness ratio, which presents a high dielectric constant of 52.7 and a large 2P_r value of up to 72.3 μC/cm² without any wake-up operation. Moreover, the HZO/ZrO₂ nanobilayer thin films demonstrate faster polarization switching speed (1.09 μs) and better fatigue performance (10⁹ cycles) compared to the conventional HZO solid solution films. The relationship between ferroelectric and dielectric properties can be harmoniously balanced through the designation. The results indicate that the HZO/ZrO₂ nanobilayer engineering strategy is quite potential to pave the way for the development of next-generation memory technologies with superior performance and reliability.

Currently, the development of dielectric ceramic capacitors is restricted by the contradiction between high efficiency and high recoverable density. Therefore, a novel strategy was designed to achieve a superior balance between them. Firstly, introducing Sr_0.85La_0.1TiO₃ can enhance the content of the weak polar phase (P4bm) to become the main component, which can optimize the relaxor behaviour and improve efficiency. Then, the electric breakdown strength was effectively enhanced by grain refinement and viscous polymer processing. Finally, a high recoverable energy density of ~5.3 J/cm³ and an excellent efficiency of ~92.2% were attained in 0.9Bi_0.5Na_0.5TiO₃-0.1Na_0.8Sr_0.1NbO₃ ceramic with the addition of 0.35Sr_0.85La_0.1TiO₃ after viscous polymer processing. The piezoelectric force microscope had been applied to prove the high activity of the polar nanoregions and finite element analysis was adopted to explain the reasons for the enhancing electric breakdown strength. In addition, this ceramic exhibits good temperature and frequency stability, and a fast discharging rate of 0.11 μs, making it a potential candidate for the actual application.

The sodium (Na) and Ce co-doped calcium bismuth titanate (CBT; CaBi₄Ti₄O₁₅) Aurivillius ceramics in a Ca_1−x(Na_0.5Ce_0.5)_xBi₄Ti₄O₁₅ (CNCBT; doping content (x) = 0, 0.03, 0.05, 0.08 and 0.12) system were synthesized by the conventional solid-state sintering method. All compositions show a single-phase orthorhombic (space group: A2₁am) structure at room temperature. The shift of the Curie point (T_C) towards lower temperatures (T) on doping results from the increased tolerance factor (t). The substitution-enhanced ferroelectric performance with large maximum polarization (P_m) and facilitated domain switching is evidenced by the developed electrical polarization–electric field (P–E) and electrical current–electric field (I–E) hysteresis loops. The piezoelectric coefficient (d₃₃ = 20.5± 0.1 pC/N) of the x = 0.12 sample is about four times larger than that of pure CBT. The improved piezoelectric properties can be attributed to the high remanent polarization (P_r) and relatively high dielectric permittivity (ε′). In addition, multi-sized (micron and sub-micron) domain structures were observed in the CNCBT ceramics by the piezoresponse force microscope (PFM). The multiple-sized ferroelectric domain structure with smaller domains is beneficial to the easy domain switching, enhanced ferroelectric performance, and improved piezoelectric properties of the CNCBT ceramics. The designed Aurivillius-phase ferroelectric ceramics with the T_C around 765 ℃ and high piezoelectric coefficient (d₃₃) are suitable for high-temperature piezoelectric applications.

Lead-free BiFeO₃-BaTiO₃ ceramics attract widespread attention over the last two decades due to their high Curie temperature (T_C) and excellent piezoelectric performance. Here, in the Nd-modified 0.67BiFeO₃-0.33BaTiO₃ ceramics, an excellent piezoelectric constant (d₃₃) of 325 pC/N was achieved by applying a novel poling method (AC-bias + DC-bias) with a high T_C of 455 ℃. In addition, an ultrahigh normalized piezoelectric strain (d₃₃* = S_max/E_max) of 808 pm/V was obtained at the normal/typical and relaxor-ferroelectrics phase boundary simultaneously with good thermal stability (Δd₃₃*(T) ≈ 20%) in the temperature range of 25–125 ℃. The piezoelectric force microscopy results show the domain miniaturization from micro to nanoscale/polar nano-regions due to local structure heterogeneity caused by Nd doping. The mechanism for the giant piezoelectric strain is attributed to the thermal quenching, nano-domains, and reverse switching of the short-range order to the long-range order under the applied electric field. The strategic design of domain engineering and a proposed model for the high piezoelectricity is successfully supported by the phenomenological relation and Gibbs free energy profile. In this work, a new lead-free single-element modified BiFeO₃-BaTiO₃ ceramics was developed by applying a synergistic approach of domain engineering and phase boundary for the high-temperature piezoelectric performance.

Core–shell structured Bi_0.5Na_0.5TiO₃KTaO₃ + x% (in mass) Li₂CO₃ ceramics were fabricated in this study. Increasing x from 0 to 2 leads to the decrease of sintering temperature from 1 175 ℃ to 1 020 ℃. The limited diffusion of Ta⁵⁺ results in chemical heterogeneities and core–shell microstructures. The Ta⁵⁺-depleted cores show the nanodomains (~10 nm), while the Ta⁵⁺-rich shells display the polar nanoregions (1–2 nm). From x = 0 to 1, the appearance of cores with nanodomains contributes to the increase of dielectric constant and maximum polarization, while the further addition of Li₂CO₃ suppresses the dielectric and polarization responses due to the reduced grain sizes and polarization coupling. The enhanced dielectric relaxation and existence of core-shell microstructure with different polarization levels help to optimize the dielectric temperature stability. The x = 2 ceramics exhibit a stable high dielectric constant ~1 400 over a wide temperature range of 20–520 ℃. More encouragingly, the ultrafine grain size and core–shell microstructure in the x = 2 ceramics greatly benefit the improvement of breakdown strength. Combined with the delayed polarization saturation and high ergodicity, a high recoverable energy density of ~5.07 J/cm³ is obtained under 44 kV/mm, with a high efficiency of ~85.17%.

Herein, a high strain of ~0.3% with a small hysteresis of 43% is achieved at a low electric field of 4 kV/mm in the highly <001>-textured 0.97(0.76Bi_0.5Na_0.5TiO₃–0.24SrTiO₃)–0.03NaNbO₃ (BNT–ST–0.03NN) ceramics with an ergodic relaxor (ER) state, leading to a large normalized strain (d₃₃^*) of 720 pm/V. The introduction of NN templates into BNT–ST induces the grain orientation growth and enhances the ergodicity. The highly <001>-textured BNT–ST–0.03NN ceramics display a pure ergodic relaxor state with coexisted ferroelectric R $\overline{3}$c and antiferroelectric P4bm polar nanoregions (PNRs) on nanoscale. Moreover, due to the incomplete interdiffusion between the NN template and BNT–ST matrix, the textured ceramics present a core–shell structure with the antiferroelectric NN core, and thus the BNT-based matrix owns more R $\overline{3}$c PNRs relative to the homogeneous nontextured samples. The high <001> crystallographic texture and more R $\overline{3}$c PNRs both facilitate the relaxor-to-ferroelectric transition, leading to the low-field-driven high strain, while the ergodic relaxor state ensures a small hysteresis. Furthermore, the d₃₃^* value remains high up to 518 pm/V at 100 ℃ with an ultra-low hysteresis of 6%.

Giant strains in (Bi_0.5Na_0.5)TiO₃ based ceramics are usually attributed to electric field induced nonpolar to polar phase transition. Whether it is an ergodic relaxor R3c/P4mm ferroelectric (FE) to long-range ordered FE phase transformation or a reversible P4bm antiferroelectric (AFE) to FE phase transition is still unclear. Herein, lead-free (0.88-x)(Bi_0.5Na_0.5)TiO₃-0.12BaTiO₃-xNaNbO₃ ceramics exhibit a composition-modulated FE tetragonal P4mm to relaxor AFE tetragonal P4bm phase transition, in which double hysteresis loop, sprout-shaped S-E curves, near-zero quasi-static d₃₃ together with a large volume change suggest the AFE characteristics of P4bm phase. An interesting finding is that the reversibility of field-induced AFE P4bm phase to FE P4mm phase transition strongly depends on the NN content, from being completely irreversible at x = 0.01–0.02, to partially reversible at x = 0.03–0.05, and finally to completely reversible at x = 0.06–0.08. It is indicated that the variation of reversibility should be attributed to the change of relative free energy caused by decreasing the FE to AFE phase transition temperature with increasing the NN content.