Achieving Large Electrocaloric Effect in a Wide Temperature Span for (Na_1/2Bi_1/2)TiO₃-Based Ceramics via the Synergic Effect of A-Site Vacancies and B-Site Complex Cations

Abstract

Probing a lead-free ferroelectric with a large electrocaloric effect (ECE) and excellent temperature stability is a challenge in developing solid-state electrocaloric refrigeration. In this work, we constructed 0.93(Na 1/2 Bi 1/2 )TiO 3 -0.07[(1 − x)-BaTiO 3 -x(Nd 2/3 □ 1/3 )(Zn 1/3 Nb 2/3 )O 3 ] (NZN100x) lead-free ferroelectric ceramics by introducing A-site vacancies and B-site complex cations simultaneously. Direct electrocaloric measurement shows that, for x = 0.14, a high ΔT of 1.17 K was obtained at 45 °C under 5 kV/mm; meanwhile, the sample x = 0.20 gives a large ΔT of 0.91 K with a 10% variation in a wide temperature span (T span ) from 40 to 80 °C. The excellent ECE performance opens up a route for designing lead-free electrocaloric materials based on the synergic effect of composition design and defect engineering. KEYWORDS: (Na 1/2 Bi 1/2 )TiO 3 , lead-free ceramics, A-site vacancy, B-site complex cation, electrocaloric effect, temperature stability

Effective Enhancement of the Ferroelectric Performance of Polar Co-Gallate MOF by Doping M2+ Ions (M = Mg, Mn, Ni) into Framework Nodes

“…What is more, the main diffraction peaks of M-doped Co-Gallate (M = Mg, Mn, and Ni) also correspond to Co-Gallate with increasing feed molar ratio of doped metal ions, which not only illustrate that M-doped Co-Gallate (M = Mg, Mn, and Ni) are successfully constructed but also demonstrate that the doping of metal ions do not change the whole crystal structures. However, the diffraction peaks of M-doped Co-Gallate at 11.44° exhibit a slight shift compared with that of Co-Gallate (Figure S1a–c), with a slight shift to smaller angles for Mg 2+ and to larger angles for Mn 2+ and Ni 2+ , indicating that the shift is related to the expansion and contraction of the unit cell due to the smaller or larger ionic size of the dopant cations, which will give rise to the framework distortion and further influence the polarity of the whole structure . Meanwhile, the real content of doped metal ions M 2+ (M = Mg, Mn, and Ni) in Co-Gallate was measured by ICP-OES, which is basically consistent with the feed molar ratio (1, 3, 5, 7, and 10%), and the results are listed in Table S1.…”

“…As shown in Figure S5a, Co-Gallate displayed a well-shaped P–E hysteresis loop with a remanent polarization ( P r ) of 0.115 μC/cm 2 and coercive field ( E c ) of 10.56 kV/cm, which is a weaker ferroelectric performance than the classic ferroelectric Rochelle salt (0.25 μC/cm 2 ). In order to improve the ferroelectric performance of Co-Gallate, a strategy of doping metal ions into the framework is adopted, which can trigger the distortion of framework and asymmetry of local charge distribution, resulting in the increase of polarizability of the overall structure and thus enhance or tune the ferroelectric performance of MOFs . Therefore, Mg 2+ ions in the second main group, with the same valence state as Co 2+ ions and an ionic radius similar to Co 2+ ions, are selected to dope into the framework of Co-Gallate in order to improve ferroelectric behaviors.…”

“…In order to improve the ferroelectric performance of Co-Gallate, a strategy of doping metal ions into the framework is adopted, which can trigger the distortion of framework and asymmetry of local charge distribution, resulting in the increase of polarizability of the overall structure and thus enhance or tune the ferroelectric performance of MOFs. 26 Therefore, Mg 2+ ions in the second main group, with the same valence state as Co 2+ ions and an ionic radius similar to Co 2+ ions, are selected to dope into the framework of Co-Gallate in order to improve ferroelectric behaviors. All Mg-doped Co-Gallate with different feed molar ratios show a P−E hysteresis loop (Figure S6), indicating their ferroelectric properties.…”

“…It can be seen from the above analysis that the ferroelectric properties of parent Co-Gallate can be effectively improved by doping Mg 2+ , Mn 2+ , and Ni 2+ ions into the framework nodes of parent Co-Gallate, which is ascribed to the framework distortion triggered by inserting metal ions into framework nodes and further increase the polarization of the overall structure. 26 The difference value of ionic radius between Co 2+ ions and M 2+ ions (M = Mg, Mn, Ni) is defined as Δr, and the larger the Δr is, the greater the framework deformation is likely to be. 37 As shown in Figure 4a, the Δr increases in the order Mg < Ni < Mn, which will cause different changes in framework distortion when M 2+ doping ions vary from Mg 2+ to Ni 2+ to Mn 2+ .…”

Excellent energy storage and discharge performances in Na1/2Bi1/2TiO3-based ergodic relaxors by enlarging the [AO12] cages

“…12 Many strategies have been put forward to lower the P r , such as constructing an ergodic relaxor 13 or super-paraelectric dielectric, 14,15 or optimizing the domain structure, 16 thus enhancing the DP. Several researchers have tried to enhance the P m by introducing A site-vacancies, thus enlarging the DP, 12,17,18 but the introduction of vacancies may lead to the lowering of the E b . 19 Hence, constructing a relaxor with enhanced local polarization without the deterioration of the E b is of great meaning to design a dielectric ceramic with excellent energy storage properties.…”