Enhanced Energy Storage Performance and Efficiency in Bi0.5(Na0.8K0.2)0.5TiO3-Bi0.2Sr0.7TiO3 Relaxor Ferroelectric Ceramics via Domain Engineering

Dielectric materials are highly desired for pulsed power capacitors due to their ultra-fast charge-discharge rate and excellent fatigue behavior. Nevertheless, the low energy storage density caused by the low breakdown strength has been the main challenge for practical applications. Herein, we report the electric energy storage properties of (1 − x) Bi0.5(Na0.8K0.2)0.5TiO3-xBi0.2Sr0.7TiO3 (BNKT-BST; x = 0.15–0.50) relaxor ferroelectric ceramics that are enhanced via a domain engineering method. A rhombohedral-tetragonal phase, the formation of highly dynamic PNRs, and a dense microstructure are confirmed from XRD, Raman vibrational spectra, and microscopic investigations. The relative dielectric permittivity (2664 at 1 kHz) and loss factor (0.058) were gradually improved with BST (x = 0.45). The incorporation of BST into BNKT can disturb the long-range ferroelectric order, lowering the dielectric maximum temperature Tm and inducing the formation of highly dynamic polar nano-regions. In addition, the Tm shifts toward a high temperature with frequency and a diffuse phase transition, indicating relaxor ferroelectric characteristics of BNKT-BST ceramics, which is confirmed by the modified Curie-Weiss law. The rhombohedral-tetragonal phase, fine grain size, and lowered Tm with relaxor properties synergistically contribute to a high Pmax and low Pr, improving the breakdown strength with BST and resulting in a high recoverable energy density Wrec of 0.81 J/cm3 and a high energy efficiency η of 86.95% at 90 kV/cm for x = 0.45.


Introduction
Materials with high energy and power have received extensive attention for highpower applications, such as microwaves, electromagnetic devices, pulsed power devices, hybrid electric vehicles, high-frequency inverters, and other energy storage devices [1][2][3]. In particular, dielectric ceramics are the most promising materials for energy storage applications due to their super-fast charge-discharge rate and excellent temperature stability compared to electrochemical energy storage devices (batteries and electrochemical capacitors) and dielectric polymers [4][5][6]. However, the dielectric capacitor's energy storage density and efficiency are much lower than those of polymers/batteries due to their low dielectric breakdown strength (DBS), which restricts their practical application in energy storage devices.
The recoverable energy density (W rec ) of a dielectric capacitor is governed by the applied electric field (E) and induced polarization (P), expressed by the following equation, Materials 2023, 16 usually estimated from the P-E loop, and it is schematically shown in Figure 1a by the shaded area with cyan color [7][8][9][10].
W rec = P max P r E dP (1) where P max and P r are the maximum polarization and remnant polarization, respectively ( Figure 1a). Energy efficiency (η) can be estimated by the following equation [8][9][10]. η = W rec W rec + W loss (2) where W loss is the hysteresis loss. According to Equation (1), the energy storage properties can be significantly enhanced by increasing the difference between P r and P max (∆P). The breakdown electric field (E BD ) is also an essential factor for energy storage; i.e., a higher DBS is responsible for a large energy storage density.
Materials 2023, 16, x FOR PEER REVIEW 2 of 12 The recoverable energy density (Wrec) of a dielectric capacitor is governed by the applied electric field (E) and induced polarization (P), expressed by the following equation, usually estimated from the P-E loop, and it is schematically shown in Figure 1a by the shaded area with cyan color [7][8][9][10]. (1) where Pmax and Pr are the maximum polarization and remnant polarization, respectively (Figure 1a). Energy efficiency (η) can be estimated by the following equation [8][9][10]. (2) where Wloss is the hysteresis loss. According to Equation (1), the energy storage properties can be significantly enhanced by increasing the difference between Pr and Pmax (ΔP). The breakdown electric field (EBD) is also an essential factor for energy storage; i.e., a higher DBS is responsible for a large energy storage density.
Perovskite-structured BNT-based ceramics exhibit a strong ferroelectric response, since Bi 3+ has a lone pair of electrons (6s 2 ), which strongly hybridizes with the oxygen 2p orbital [38]. Furthermore, the formation of highly dynamic polar nano-regions (PNRs) are facilitated by local random fields induced by valency differences and compositional inhomogeneity [39,40]. Moreover, the relaxor behavior of the material can be improved by adding another phase with a similar perovskite to form a solid solution or modifying the base compound with a suitable dopant, which enables slim P-E loops [41]. Sayyed et al. [42] investigated the local structural deformation and dielectric anomalies near the morphotropic phase boundary (MPB) of (1 − x) Na 0.5 Bi 0.5 TiO 3 -xSrTiO 3 ceramics. The ferroelectric response of (1 − x) Na 0.5 Bi 0.5 TiO 3 -SrTiO 3 -xAgNbO 3 ceramics is similar to the antiferroelectric response and improved energy storage performance [43]. Shi et al. [44] reported that Zr-and Sm-doped 0.74Na 0.5 Bi 0.5 TiO 3 -0.26SrTiO 3 ceramics significantly enhanced energy storage performance and the DBS. Bi 0.2 Sr 0.7 TiO 3 (BST) exhibits strong polarization and a widephase transition temperature with diffused dielectric maxima. It was incorporated into BNT ceramics, suppressing the field-generated ferroelectric phase and achieving a large P max and small P r [45,46]. Recently, Li et al. reported a synergistic approach to enhance the energy storage response in BNT-based RFEs by introducing PNRs and lowering the transition temperature by stabilizing the AFE responses at low temperatures [9].
In this work, we investigate a domain engineering process to improve the energy storage performance by modifying Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 (BNKT) RFEs with BST, since Bi 0.5 (Na 1−x K x ) 0.5 TiO 3 exhibits a stronger ferroelectric response with relaxor behavior at the MPB at x = 0.16-0.2 [47,48] than pure BNT. It is revealed that the addition of BST can disturb the long-range ferroelectric order and transform the ferroelectric microdomains of BNKT into highly dynamic PNRs. This results in a macroscopic ferroelectric to relaxor ferroelectric transition, as schematically illustrated in Figure 1b. The favorable relaxor ferroelectric state formed by the domain engineering method simultaneously produces a large P max and reduced P r , which facilitates the enhancement in DBS with the BST, resulting in high energy density and high efficiency of the BNKT-BST RFEs. After the slurries were dried at 120 • C, the mixture of BNKT and BST powders was calcined at 800 • C and 950 • C for 2 h and 3 h, respectively, to form a pure phase of Bi 0.5 (Na 0.8 K 0.2 ) 0.5 TiO 3 and Bi 0.2 Sr 0.7 TiO 3 . Both BNKT and BST calcined powders were mixed and ball-milled for 12h to prepare a BNKT-BST composition. Further, these powders were granulated with 5 wt.% polyvinyl alcohol (Sigma-Aldrich, 99%, St. Louis, MI, USA,) and uniaxially pressed into disks, at a pressure of 10 MPa, of 10 mm diameter and~0.5 mm thickness, followed by sintering at 1100 • C for 3 h. To perform electrical measurements, a silver paste (ELCOAT, Electroconductives) was coated on both sides of the sintered disks.

Materials and Methods
The phase formation of the BNKT-BST ceramic samples was examined using an X-ray diffractometer (Rigaku, Tokyo, Japan, TTRAX III 18 kW) with monochromatic Cu-Kα radiation (λ = 1.5406 Å). Raman spectra were recorded using a Raman spectrometer (JOBIN YVON, Oberursel, Germany, LABRAM HR800) with a laser wavelength of 532.06 nm. Surface morphology was investigated using a field emission scanning electron microscope (FESEM) (JEOL, Tokyo, Japan, JSM-7610F). Temperature-and frequency-dependent dielectric properties were measured from room temperature (RT) to 450 • C and 1 kHz-1 MHz using an impedance analyzer (Hewlett Packard, Palo Alto, CA, USA, 4294A). P-E, I-E loops, and fatigue behavior were measured using a ferroelectric tester (AixACCT Systems GmbH, Aachen, Germany, TF Analyzer 2000).  Figure 2a shows the X-ray diffraction (XRD) patterns of BNKT-BST ceramics (x = 0. 15-0.50) in the 2θ range of 20-70 • . All of the samples revealed a rhombohedral and tetragonal crystal structure, indicating the diffusion of BST into BNKT and the formation of BNKT-BST as a homogeneous solid solution. At RT, the BNKT system exhibits a rhombohedral and tetragonal crystal structure near MPB at x = 0.16-0.2 [47,48]. The formation of MPB in BNKT-BST ceramics is confirmed by the splitting of the (021)/(111) and (122)/(211) peaks at 2θ around 40 • and 58 • , respectively, which is shown in Figure 2b. Similar splitting and formation of MPB were observed in Bi 0.5 (Na 1-x K x ) 0.5 TiO 3 ]-BiAlO 3 [49], Bi 0.5 Na 0.5 TiO 3 -Bi 0.5 K 0.5 TiO 3 -Bi 0.5 Li 0.5 TiO 3 [50], and (Bi 0.5 Na 0.5 )TiO 3 -(Bi 0.5 K 0.5 )TiO 3 -BaTiO 3 [51] ceramics. In addition, both the (021) and (122) peaks shifted slightly toward lower angles with increasing BST into BNKT, demonstrating enhanced lattice parameters ( Figure 2b). The enhancement in lattice parameters can be attributed to the ionic radius of Sr 2+ (1.44 Å), which is larger than that of Bi 3+ (1.36 Å), Na + (1.39 Å), and K + (1.38 Å), respectively, at the A-site [52][53][54].  Figure 2a shows the X-ray diffraction (XRD) patterns of BNKT-BST ceramics (x = 0.15-0.50) in the 2θ range of 20-70°. All of the samples revealed a rhombohedral and tetragonal crystal structure, indicating the diffusion of BST into BNKT and the formation of BNKT-BST as a homogeneous solid solution. At RT, the BNKT system exhibits a rhombohedral and tetragonal crystal structure near MPB at x = 0.16 -0.2 [47,48]. The formation of MPB in BNKT-BST ceramics is confirmed by the splitting of the (021)/(111) and (122)/(211) peaks at 2θ around 40° and 58°, respectively, which is shown in Figure 2b. Similar splitting and formation of MPB were observed in Bi0.5(Na1−xKx)0.5TiO3]-BiAlO3 [49], Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3-Bi0.5Li0.5TiO3 [50], and (Bi0.5Na0.5)TiO3-(Bi0.5K0.5)TiO3-BaTiO3 [51] ceramics. In addition, both the (021) and (122) peaks shifted slightly toward lower angles with increasing BST into BNKT, demonstrating enhanced lattice parameters (Figure 2b). The enhancement in lattice parameters can be attributed to the ionic radius of Sr 2+ (1.44 Å), which is larger than that of Bi 3+ (1.36 Å), Na + (1.39 Å), and K + (1.38 Å), respectively, at the A-site [52][53][54].    and (iv) the modes above 700 cm −1 are related to the A1 and E (longitudinal optical) overlapping modes [55]. The modes appearing at 124-172 cm −1 and 768 cm −1 are shifted to the higher wavenumbers of 128-189 cm −1 and 779 cm −1 with BST, associated with the A-site and A1 + E vibrations caused by A-site disorder. Such a disorder is induced by the incorporation of BST (Bi 3+ and Sr 2+ ) into the BNKT (Bi 3+ , Na + , and K + ) system [56]. In addition, a noticeable change at 250 and 320 cm −1 shifted toward a lower wavenumber of 233 and 305 cm −1 with BST, which is caused by an increase in the B-site disorder in the BNKT-BST system [53]. Moreover, these modes are slightly broadened, confirming the disturbance of the long-range ferroelectric order and the formation of highly dynamic PNRs, improving the relaxor characteristics of BNKT-BST [44]. This result is consistent with the XRD and electrical properties presented in Sections 3.2 and 3.3. Figure 3 shows FESEM images of the BNKT-BST ceramics. All of the ceramics display rectangular-shaped grains, which are homogeneously distributed. Figure 3d clearly shows that the x = 0.45 composition exhibits a highly dense microstructure and is more compact with smaller grains, as compared to the other samples of BNKT-BST (x < 0.45 and x = 0.50). To prove that all of the samples are homogeneous and highly dense, the density of sintered BNKT-BST ceramic samples was calculated using the Archimedes principle. It increased with BST from 5.58 g/cm 3 to 5.73 g/cm 3 for x = 0.15 to 0.45 and further decreased (5.54 g/cm 3 ) for x = 0.50. The calculated relative density of BNKT-BST ranged from 94.96% to 98.17% of the theoretical density [57], confirming that these samples are homogeneous and highly dense. Further, the average grain size of the BNKT-BST ceramics was estimated using Image-J software (Wayne Rasband and contributors, National Institutes of Health, USA, ImageJ 1.53t) via the linear intercept method and found to be 1.37 µm for the x = 0.15 composition; it gradually enhanced to 1.6 µm with the incorporation of BST. Grain size enhancement is caused by the generation of oxygen vacancies by Sr 2+ entering the perovskite of BNKT and being substituted at the A-site of Bi 3+ , Na + , and K + [58]. Previous reports have investigated that the fine grain size with a homogeneous and dense microstructure can withstand higher electric fields, leading to high DBS, and improve energy storage performance [59,60]. incorporation of BST (Bi 3+ and Sr 2+ ) into the BNKT (Bi 3+ , Na + , and K + ) system [56]. In addition, a noticeable change at 250 and 320 cm −1 shifted toward a lower wavenumber of 233 and 305 cm −1 with BST, which is caused by an increase in the B-site disorder in the BNKT-BST system [53]. Moreover, these modes are slightly broadened, confirming the disturbance of the long-range ferroelectric order and the formation of highly dynamic PNRs, improving the relaxor characteristics of BNKT-BST [44]. This result is consistent with the XRD and electrical properties presented in Sections 3.2 and 3.3. Figure 3 shows FESEM images of the BNKT-BST ceramics. All of the ceramics display rectangular-shaped grains, which are homogeneously distributed. Figure 3d clearly shows that the x = 0.45 composition exhibits a highly dense microstructure and is more compact with smaller grains, as compared to the other samples of BNKT-BST (x < 0.45 and x = 0.50). To prove that all of the samples are homogeneous and highly dense, the density of sintered BNKT-BST ceramic samples was calculated using the Archimedes principle. It increased with BST from 5.58 g/cm 3 to 5.73 g/cm 3 for x = 0.15 to 0.45 and further decreased (5.54 g/cm 3 ) for x = 0.50. The calculated relative density of BNKT-BST ranged from 94.96% to 98.17% of the theoretical density [57], confirming that these samples are homogeneous and highly dense. Further, the average grain size of the BNKT-BST ceramics was estimated using Image-J software (Wayne Rasband and contributors, National Institutes of Health, USA, ImageJ 1.53t) via the linear intercept method and found to be 1.37 µm for the x = 0.15 composition; it gradually enhanced to 1.6 µm with the incorporation of BST. Grain size enhancement is caused by the generation of oxygen vacancies by Sr 2+ entering the perovskite of BNKT and being substituted at the A-site of Bi 3+ , Na + , and K + [58]. Previous reports have investigated that the fine grain size with a homogeneous and dense microstructure can withstand higher electric fields, leading to high DBS, and improve energy storage performance [59,60].    Figure 4a displays the frequency variation in the relative dielectric permittivity (ε r ) (solid line) and loss factor (tan δ) (dot line) of BNKT-BST ceramic capacitors, measured at RT in the range of 1 kHz to 1 MHz. The sample x = 0.15 displayed a higher ε r of 1481 and tan δ of 0.231 at 1 kHz than pure BNKT (ε r of 1273 and tan δ of 0.047 at 1 kHz), as reported in our previous report [48]. These ε r values gradually enhanced to 2664, and the tan δ values reduced to 0.058 for the x = 0.45 sample (Figure 4b). The enhancement in the dielectric properties is attributed to the incorporation of BST into BNKT and the dense microstructure. dielectric properties is attributed to the incorporation of BST into BNKT and the dense microstructure.   [48], and this is similar to the 0.74 Na0.5Bi0.5TiO3-0.26 SrTiO3 ceramics reported by Shi et al. [44]. The incorporation of BST into BNKT can disturb the long-range ferroelectric order, resulting in a lowered Tm. This lower Tm leads to the formation of highly dynamic PNRs due to the mismatch of the ionic radius at the A-site of BNKT-BST. In addition, the Tm shifted toward higher temperatures, and dielectric peaks diffused with an increase in frequency. This frequency dispersion with a diffuse phase transition reveals typical relaxor ferroelectric characteristics [61,62]. The degree of the relaxor characteristics was determined using the modified Curie-Weiss law via the following equation [63,52].

Dielectric Properties and Relaxor Behavior
where is the maximum relative dielectric permittivity at the maximum temperature Tm, T is the temperature, γ is the degree of relaxation, and C is the Curie constant. Generally, the γ value is 1 for normal ferroelectrics and between 1 and 2 for relaxor ferroelectrics [63]. The insets of Figure 4c,d show the log-log plots of (1/ 1/ ) vs. (T -Tm) of BNKT-BST for x = 0.15 and 0.45, measured at 1 MHz. The value of γ slightly increased from 1.80 to 1.83, proving that there is an increase in relaxor behavior with BST from x = 0.15 to 0.45, leading to an increase in the energy storage performance. This is consistent with Raman's results and previous reports [4][5][6].   [48], and this is similar to the 0.74 Na 0.5 Bi 0.5 TiO 3 -0.26 SrTiO 3 ceramics reported by Shi et al. [44]. The incorporation of BST into BNKT can disturb the long-range ferroelectric order, resulting in a lowered T m . This lower T m leads to the formation of highly dynamic PNRs due to the mismatch of the ionic radius at the A-site of BNKT-BST. In addition, the T m shifted toward higher temperatures, and dielectric peaks diffused with an increase in frequency. This frequency dispersion with a diffuse phase transition reveals typical relaxor ferroelectric characteristics [61,62]. The degree of the relaxor characteristics was determined using the modified Curie-Weiss law via the following equation [52,63].
where ε m r is the maximum relative dielectric permittivity at the maximum temperature T m , T is the temperature, γ is the degree of relaxation, and C is the Curie constant. Generally, the γ value is 1 for normal ferroelectrics and between 1 and 2 for relaxor ferroelectrics [63]. The insets of Figure 4c,d show the log-log plots of (1/ε r − 1/ε m r ) vs. (T − T m ) of BNKT-BST for x = 0.15 and 0.45, measured at 1 MHz. The value of γ slightly increased from 1.80 to 1.83, proving that there is an increase in relaxor behavior with BST from x = 0.15 to 0.45, leading to an increase in the energy storage performance. This is consistent with Raman's results and previous reports [4][5][6]. BNKT-BST (x = 0.15) sample exhibits a typical ferroelectric (FE) characteristic, displaying high remnant polarization P r of 19.89 µC/cm 2 , high maximum polarization P max of 31.46 µC/cm 2 , and a high coercive field E c of 16.66 kV/cm. These values, listed in Table 1, gradually decreased, whereas the E max or E BD increased from 57.42 kV/cm to 90 kV/cm with the incorporation of BST (x = 0.45), which is favorable for high energy storage density (Figure 5f). It is evident that the two peaks in the I-E curves (x ≥ 0.30) and slim P-E loops are attributed to the formation of highly dynamic PNRs, which can commonly be seen in RFEs [64]. In general, the P-E loops present in normal FEs are due to the macroscopic domain wall motion, while in RFEs, highly dynamic PNRs exist instead of macrodomains, resulting in slim P-E loops [19].   Table 1, gradually decreased, whereas the Emax or EBD increased from 57.42 kV/cm to 90 kV/cm with the incorporation of BST (x = 0.45), which is favorable for high energy storage density ( Figure  5f). It is evident that the two peaks in the I-E curves (x ≥ 0.30) and slim P-E loops are attributed to the formation of highly dynamic PNRs, which can commonly be seen in RFEs [64]. In general, the P-E loops present in normal FEs are due to the macroscopic domain wall motion, while in RFEs, highly dynamic PNRs exist instead of macrodomains, resulting in slim P-E loops [19].  Further, the Wrec was calculated via Equation (1) from P-E loops, which are shown in Figure 6 (cyan shaded area). The Wloss is calculated by the enclosed area of the P-E loops in the first quadrant (magenta shaded area), and η is calculated by Equation (2); they are listed in Table 1. The Wrec values gradually increased, the Wloss values decreased with the substitution of BST, and the composition x = 0.45 displays a high energy density of 0.81 J/cm 3 at an EBD of 90 kV/cm and high energy efficiency of 86.95% (Figure 6f). The improvement in the energy storage performance is achieved via the domain engineering method by modifying BNKT with BST. It can be understood that the substitution of BST can transform the ferroelectric microdomains of BNKT into highly dynamic PNRs, resulting in a macroscopic FE to RFE transition. This domain evolution and transformation of FE to RFE transition in the present samples is schematically shown in Figure 1b. The highly dynamic  Further, the W rec was calculated via Equation (1) from P-E loops, which are shown in Figure 6 (cyan shaded area). The W loss is calculated by the enclosed area of the P-E loops in the first quadrant (magenta shaded area), and η is calculated by Equation (2); they are listed in Table 1. The W rec values gradually increased, the W loss values decreased with the substitution of BST, and the composition x = 0.45 displays a high energy density of 0.81 J/cm 3 at an E BD of 90 kV/cm and high energy efficiency of 86.95% (Figure 6f). The improvement in the energy storage performance is achieved via the domain engineering method by modifying BNKT with BST. It can be understood that the substitution of BST can transform the ferroelectric microdomains of BNKT into highly dynamic PNRs, resulting in a macroscopic FE to RFE transition. This domain evolution and transformation of FE to RFE transition in the present samples is schematically shown in Figure 1b. The highly dynamic PNRs induced large P max and low P r , which improved the DBS with the incorporation of BST, resulting in high energy storage density and high energy efficiency of the BNKT-BST RFEs [65]. The obtained W rec and η of 0.55 BNKT-0.45 BST are comparable/superior to other lead-free RFEs and are promising for energy storage capacitors [64][65][66][67][68][69].

FE-RFE Transformation, Domain Evolution, and Energy Storage Performance
PNRs induced large Pmax and low Pr, which improved the DBS with the incorporation of BST, resulting in high energy storage density and high energy efficiency of the BNKT-BST RFEs [65]. The obtained Wrec and η of 0.55 BNKT-0.45 BST are comparable/superior to other lead-free RFEs and are promising for energy storage capacitors [64,[65][66][67][68][69]. Electrical fatigue endurance is an important property necessary for energy storage applications. Therefore, the fatigue behavior of BNKT-BST ceramic capacitors was measured up to 10 6 electric cycles at a frequency of 10 Hz under an electric field of 90 kV/cm. Figure 7 shows the unipolar P-E loops of BNK-BST (x = 0.45) and corresponding Wrec (square line) and η (circle line) values measured after various electric cycles (black, red and blue colour P-E loops measured at 10 0 and 10 3 and 10 6 , respectively, as shown in inset of Figure 7). It is observed that the slender P-E loops are without significant change, revealing an excellent fatigue-free response and negligible variations in Wrec and η.

Conclusions
The domain-engineered relaxor ferroelectric BNKT-BST lead-free ceramics were fabricated by a solid-state reaction method and demonstrated structural, microstructural, dielectric, and ferroelectric properties in detail. XRD, Raman spectra, and FESEM studies Electrical fatigue endurance is an important property necessary for energy storage applications. Therefore, the fatigue behavior of BNKT-BST ceramic capacitors was measured up to 10 6 electric cycles at a frequency of 10 Hz under an electric field of 90 kV/cm. Figure 7 shows the unipolar P-E loops of BNK-BST (x = 0.45) and corresponding W rec (square line) and η (circle line) values measured after various electric cycles (black, red and blue colour P-E loops measured at 10 0 and 10 3 and 10 6 , respectively, as shown in inset of Figure 7). It is observed that the slender P-E loops are without significant change, revealing an excellent fatigue-free response and negligible variations in W rec and η.
PNRs induced large Pmax and low Pr, which improved the DBS with the incorporation of BST, resulting in high energy storage density and high energy efficiency of the BNKT-BST RFEs [65]. The obtained Wrec and η of 0.55 BNKT-0.45 BST are comparable/superior to other lead-free RFEs and are promising for energy storage capacitors [64,[65][66][67][68][69]. Electrical fatigue endurance is an important property necessary for energy storage applications. Therefore, the fatigue behavior of BNKT-BST ceramic capacitors was measured up to 10 6 electric cycles at a frequency of 10 Hz under an electric field of 90 kV/cm. Figure 7 shows the unipolar P-E loops of BNK-BST (x = 0.45) and corresponding Wrec (square line) and η (circle line) values measured after various electric cycles (black, red and blue colour P-E loops measured at 10 0 and 10 3 and 10 6 , respectively, as shown in inset of Figure 7). It is observed that the slender P-E loops are without significant change, revealing an excellent fatigue-free response and negligible variations in Wrec and η.

Conclusions
The domain-engineered relaxor ferroelectric BNKT-BST lead-free ceramics were fabricated by a solid-state reaction method and demonstrated structural, microstructural, dielectric, and ferroelectric properties in detail. XRD, Raman spectra, and FESEM studies

Conclusions
The domain-engineered relaxor ferroelectric BNKT-BST lead-free ceramics were fabricated by a solid-state reaction method and demonstrated structural, microstructural, dielectric, and ferroelectric properties in detail. XRD, Raman spectra, and FESEM studies revealed the formation of a rhombohedral-tetragonal phase, highly dynamic PNRs, and dense microstructure. The dielectric properties were improved with BST, and a high ε r of 2664 and low tan δ of 0.058 at 1 kHz were obtained for the x = 0.45 composition. The incorporation of BST into BNKT can disturb the long-range ferroelectric order, causing lowered T m and the formation of highly dynamic PNRs. In addition, the T m shifts toward a high temperature with frequency and diffuse phase transition, indicating relaxor ferroelectric characteristics of BNKT-BST ceramics, and is confirmed via the modified Curie-Weiss