Bismuth sodium titanate-based (Bi_0.5Na_0.5TiO₃, BNT) lead-free piezoelectric ceramics exhibit significant potential for precision actuation because of their large electrostrain. However, the inherent trade-off between high electrostrain performance and temperature stability hinders their practical application. This study addresses this challenge by developing a series of Bi_0.47Na_0.47Ba_0.06Ti_1−xHf_xO₃ (BNBT-100xH) ceramics via a B-site Hf⁴⁺ doping strategy enabling synergistic regulation of the phase boundary and domain state. The optimized BNBT-3H composition (x = 0.03) features a morphotropic phase boundary (MPB) comprising coexisting rhombohedral (R3c, 51%) and tetragonal (P4bm, 49%) phases, alongside a unique coexistence domain structure of ferroelectric macrodomains and relaxor nanodomains (~100 nm). This microstructural design achieves an ultrahigh bipolar electrostrain of up to 0.6% (d₃₃^* = 500 pm/V), along with an ultralow temperature fluctuation of only 16.7% over a wide temperature range of 25–150 °C. Notably, the electrostrain at 150 °C decreases by only 4% compared with that at room temperature, demonstrating excellent thermal stability and overall performance superior to those of other lead-free systems. Through multiscale characterization, the origin of the high electrostrain is confirmed to stem from an electric field-induced reversible relaxor–ferroelectric phase transition, facilitated by the flattened energy landscape at the critical rhombohedral–tetragonal phase boundary. Simultaneously, the exceptional thermal stability arises from the thermal-electric driven dynamic equilibrium within the multiphase nanodomain structure. This work not only provides a high-performance material candidate for broad-temperature-range precision actuators but also offers novel insights into optimizing functional ceramics through precise microstructure control.

The ferroelectric cooling technology based on electrocaloric effect (ECE) has broad application scenarios in solid-state refrigeration due to its various advantages including high refrigeration efficiency, environmental friendliness, convenient operation and easy miniaturization. The ECE refers to the isothermal entropy change (ΔS) or adiabatic temperature change (ΔT) induced by the change of polar dipole state in ferroelectric and other polar materials under external electric fields, which can be classified as positive and negative ECEs. The positive (negative) ECE refers to the phenomenon that the ferroelectrics exude heat when the electric field is applied (removed) and absorb heat when the electric field is removed (applied). The process of absorbing heat from external environment of positive and/or negative ECE can be used in ferroelectric cooling. In addition to the traditional approach of optimizing the pyroelectric coefficient and breakdown field strength, the combination of positive and negative ECE is also considered an effective means to enhance the cooling capacity of ferroelectric materials. Due to short research history, few suitable materials and unclear physical mechanism, the usage of negative ECE is limited in the practical applications.

Firstly, based on the development of ECE, the relevant research progress on ECE cooling materials, devices and theories are introduced. Secondly, the physical mechanism of negative ECE are summarized. Both phase transition and domain rotation can induce negative ECE. In ferroelectric single crystals and antiferroelectric materials, the negative ECE can be induced by the phase transition from low temperature phase (low entropy) to high temperature phase (high entropy) under the external electric field. While, for some ferroelectric materials with rectangular hysteresis loops, polarization disorder induced by a reverse electric field can also produce smaller negative ECE. Thirdly, we discuss the effective strategy for the combination of positive and negative ECEs to enhance the cooling performance in ferroelectric materials. They confirm the feasibility of combining positive and negative ECEs, but how to reasonably regulate the negative ECE to coexist and alternate with the positive ECE at the same or similar temperatures is still an urgent problem. Finally, the perspectives and prospects of future research for the negative ECE are presented.

This review can provide new ideas and directions for the researches of negative ECE and the improvement of the cooling capability of ferroelectric materials, which can promote the study of ECE