Giant intrinsic electrocaloric effect in ferroelectrics by local structural engineering

Abstract

The electrocaloric effect of ferroelectrics holds great promise for solid-state cooling, potentially replacing traditional vapor-compression refrigeration systems. However, achieving adequate electrocaloric cooling capacity at room temperature remains a formidable challenge due to the need for a high intrinsic electrocaloric effect. While barium titanate ceramic exhibits a pronounced electrocaloric effect near its Curie temperature, typical chemical modifications to enhance electrocaloric properties at room temperature often reduce this intrinsic electrocaloric effect. Herein, a structural design is introduced for barium titanate-based ceramics by incorporating isovalent cations. This leads to a well-ordered local structure that decreases the Curie temperature to room temperature while preserving a sharp phase transition, enabling a large dielectric constant and tunable polarization. This design achieves a remarkable electrocaloric strength of ~1.0 K·mm/kV, surpassing previous reports. Atomic-resolution structural analyses reveal that the presence of multiscale nanodomains (from ~10 nm to >100 nm), and the dipole polarization distribution with gradual dipole rotation enable rapid phase transition and facile polarization rotation, accounting for the giant electrocaloric response. This work provides a strategy for achieving a strong intrinsic electrocaloric effect in ferroelectrics near room temperature and offers key insights into the microstructure landscapes driving this enhanced electrocaloric effect.

Fig. 1. Design strategy for high intrinsic electrocaloric (EC) effect in ferroelectrics near room temperature.

a Schematic of perovskite-type ferroelectrics and the spontaneous polarization (P_s) vector. b Schematic illustration of the EC refrigeration cycle based on the EC effect. c EC effect involving entropy change and temperature change induced by polarization change. Schematic diagrams illustrating the local lattice structure, mesoscopic domain configuration, and the evolution of macroscopic permittivity, ferroelectric polarization, and EC effect across different phase transitions: d pure barium titanate (BT), e generally modified BT with a diffuse phase transition near room temperature, and f a newly modified BT that achieves a sharp phase transition, high polarization (change) and permittivity, and a giant intrinsic EC effect is expected near room temperature. g The density functional theory (DFT) theoretical estimation of lattice distortion after incorporating different modified ions at A (Ba²⁺) or B (Ti⁴⁺) sites in tetragonal BT. Here, the coordinate of the Ba element or the Ti element is assumed to be (x, y, z), the center coordinate of the oxygen octahedron is (x’, y’, z’), and Δy = y-y’ and Δz = z-z’. The Δy is perpendicular to the P_s direction, demonstrating the relative angle of lattice distortion, and Δz is parallel to the P_s direction, demonstrating the relative magnitude of lattice distortion.

Fig. 2. Phase transitions and electrical & electrocaloric properties.

a Temperature-dependent dielectric constant of BT and BST-xBSS ceramics. b Temperature-dependent Raman spectra for BST-0.08BSS. c Comparison of room-temperature maximum polarization (P_m), remnant polarization (P_r), coercive field (E_c), dielectric constant (ε_r), and longitudinal piezoelectric coefficient (d₃₃) between BT and BST-0.08BSS. d Polarization-electric field (P–E) loops, e polarization change rate with temperature (∂P/∂T), f electrocaloric strength (ΔT/ΔE), and g electric field-induced heat flow curves (near room temperature) for BST-0.08BSS. h Comparison of ΔT/ΔE at room temperature between BT and BST-0.08BSS. i Comparison of ΔT/ΔE between this work and other lead-free and some typical lead-based ferroelectric ceramics near room temperature.

Fig. 3. Analyses of dielectric change rate, in-situ synchrotron X-ray diffraction, domain structure, and elements mapping.

a Temperature dependence of dielectric change rate with temperature (∂ε_r/∂T) at the low-temperature side of the Curie temperature for BST-xBSS. b Comparison of ∂ε_r/∂T, −∂P/∂T, and ΔT/ΔE at the low-temperature side of the Curie temperature for BST-0.08BSS. c In-situ synchrotron X-ray diffraction patterns of (111) and (200) reflections under 0 kV/cm (unpoled state), applying the electric field of 3 times the E_c (poling state), and 0 kV/cm (poled state), and the evolution of patterns intensity for BST-0.08BSS. d Transmission electron microscopy (TEM) images showing multiscale domain structure with domain widths from several nanometers to hundreds of nanometers. e Statistics of the width of representative domains marked by solid white lines in the TEM images. f Microscopic element mappings of O, Ba, Ti, Sn, and Sr elements.

Fig. 4. Atomic-scale structure and local dipole polarization distribution obtained by scanning transmission electron microscopy along the [100] zone axis for BST-0.08BSS.

a Relative intensity of the A-site ions. b Relative intensity of the B-site ions. c Statistics of intensity distribution for the A-site ions. d Statistics of intensity distribution for the B-site ions. e Calculated dipole vectors. f Distribution of dipole vectors for the amplified region corresponding to the green frame zone (The color difference of dipoles indicates the relative dipole intensity or angle). g Statistic mapping of relative dipole intensity (based on the Ti-O displacement) corresponding to the green frame zone in Supplementary Fig. S13. h Statistic distribution of relative dipole intensity corresponding to Supplementary Fig. S13. i Statistic mapping of relative dipole angle corresponding to the region shown in Fig. 4f. j Statistic distribution of relative dipole angle corresponding to the Supplementary Fig. S15.