(Bi0.5Na0.5)TiO3 ferroelectric ceramics: Achieving high depolarization temperature and improved piezoelectric properties

(Bi_1/2Na_1/2)TiO₃-based materials have received much attention due to large electro-strain and high piezoelectric constant (d₃₃), but the tough issue is that the existence of inherent depolarization temperature (T_d) limits the temperature stability and application temperature range. Previously, reports about the formation of BNT/oxide (i.e., ZnO, Al₂O₃) composites thought that T_d can be deferred to a higher temperature and then thermal depolarization improves. However, the deferred T_d of BNT/oxide composites is limited, accompanied by a low d₃₃. Here, we design the {[Bi_0.5(Na_0.8K_0.2)_0.5]_1-xPb_x}TiO₃ ceramics, leading to a big shift of T_d from 77 ℃ to 390 ℃. Large d₃₃ (140 pC/N) and high T_d (∼263 ℃) can be simultaneously achieved for the sample with Pb=0.05, and T_d could be further deferred higher (390 ℃) for Pb=0.20. The off-centre displacement of Pb induced by Pb-O hybridization in the PbO₁₂ polyhedron and ferroelectric order stabilized by the addition of Pb can provide the driving force to strengthen the ferroelectric order, and then promote the thermal stability.     

Enhanced piezoelectric properties and low electrical conductivity of Ce-doped Bi7Ti4.5W0.5O21 intergrowth piezoelectric ceramics

New types of Ce-doped Ce_xBi_7-xTi_4.5W_0.5O₂₁ (BTW-BIT-xCe) Aurivillius intergrowth ceramics with high Curie temperatures were synthesized to improve the piezoelectric performances as well as the conduction behaviour, and these ceramics exhibit great potential for high-temperature lead-free piezoelectric applications. The crystal structure, electrical properties and conduction behaviour of BTW-BIT-xCe samples were analysed thoroughly. The XRD patterns combined with Rietveld refinements of the patterns showed that the crystal structure transformed from orthorhombic structure towards pseudo-tetragonal structure with increasing CeO₂ dopant, indicating that a higher symmetry was obtained. The dielectric properties of Ce-doped samples were improved, accompanied by a significant drop in the dielectric loss and a slight decreased Curie temperature (705 °C–683 °C). An enhanced piezoelectric constant d₃₃ of 25.3 pC/N was obtained in BTW-BIT-0.12Ce, which may be attributed to a common decrease in the electrical conductivity and coercive field. Besides, a low electrical conductivity of 2 × 10^-6 S/cm at 540 °C was achieved in the same component owing to a decreased concentration of the oxygen vacancies, which was verified by analyses on XPS spectra. The above results indicate that Ce-doped BTW-BIT samples have great development potential for high temperature piezoelectric applications.     

Enhanced piezoelectric properties and electrical resistivity in W/Cr co-doped CaBi2Nb2O9 high-temperature piezoelectric ceramics

W/Cr co-doped Aurivillius-type CaBi₂Nb_2-x(W_2/3Cr_1/3)_xO₉ (CBN) (x?=?0.025, 0.050, 0.075, 0.100, and 0.150) piezoelectric ceramics were prepared by the conventional solid-state reaction method. The crystal structure, microstructure, dielectric properties, piezoelectric properties, and electrical conductivity of these ceramics were systematically investigated. After optimum W/Cr modification, the CBN ceramics showed both high d₃₃ and T_C. The ceramic with x?=?0.1 showed a remarkably high d₃₃ value of ~15 pC/N along with a high T_C of ~931?°C. Moreover, the ceramic also showed excellent thermal stability evident from the increase in its planar electromechanical coupling factor k_p from 8.14% at room temperature to 11.04% at 600?°C. After annealing at 900?°C for 2?h, the ceramic showed a d₃₃ value of 14?pC/N. Furthermore, at 600?°C, the ceramic also showed a relatively high resistivity of 4.9?×?10⁵ Ω?cm and a low tanδ of 9%. The results demonstrated the potential of the W/Cr co-doped CBN ceramics for high-temperature applications. We also elucidated the mechanism for the enhanced electrical properties of the ceramics.     

Significantly enhanced dc electrical resistivity and piezoelectric properties of Tb-modified CaBi2Nb2O9 ceramics for high-temperature piezoelectric applications

Bismuth layer–structured ferroelectric calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) is considered to be one of the most potential high-temperature piezoelectric materials due to its high Curie temperature T_c of ∼940°C, but the drawbacks of low electrical resistivity at elevated temperature and low piezoelectric performance limit its applications as key electronic components at high temperature (HT). Herein, we report significantly enhanced dc electrical resistivity and piezoelectric properties of CBN ceramics through rare-earth element Tb ions compositional adjustment. The nominal compositions of Ca_1−xTb_xBi₂Nb₂O₉ (abbreviated as CBN-100xTb) have been fabricated by conventional solid-state reaction method. The composition of CBN-3Tb exhibits a significantly enhanced dc electrical resistivity of 1.97 × 10⁶ Ω cm at 600°C, which is larger by two orders of magnitude compared with unmodified CBN. The donor substitutions of Tb³⁺ ions for Ca²⁺ ions reduce the oxygen vacancy concentrations and increase the band-gap energy, which is responsible for the enhancement of dc electric resistivity. The temperature-dependent dc conduction properties reveal that the conduction is dominated by the thermally activated oxygen vacancies in the low-temperature region (200–350°C) and by the intrinsic conduction in the HT region (350–650°C). The CBN-3Tb also exhibits enhanced piezoelectric properties with a high piezoelectric coefficient d₃₃ of ∼13.2 pC/N and a high T_c of ∼966°C. Moreover, the CBN-3Tb exhibits good thermal stabilities of piezoelectric properties, remaining 97% of its room temperature value after annealing at 900°C. These properties demonstrate the great potentials of Tb-modified CBN for high-temperature piezoelectric applications.     

Enhanced piezoelectric properties of Aurivillius-type sodium lanthanum bismuth titanate (Na0.5La0.5Bi4Ti4O15) by B-site manganese modification

The Aurivillius-type bismuth layer-structured ferroelectrics (BLSFs) sodium lanthanum bismuth titanate (Na_0.5La_0.5Bi₄Ti₄O₁₅, NLBT) polycrystalline ceramics with 0.0–0.4 wt% MnO₂ were synthesized using conventional solid-state processing. Phase analyses were performed by X-ray powder diffraction (XRPD), and the microstructural morphology was assessed by scanning electron microscopy (SEM). The dielectric and piezoelectric properties of the manganese-modified NLBT ceramics were investigated in detail. The results show that manganese is very effective in promoting the piezoelectric activities of NLBT ceramics, and the reasons for piezoelectric activities enhancement by manganese modification are explained. The NLBT ceramics modified with 0.2 wt% MnO₂ (NLBT-Mn2) possess good piezoelectric properties, with a piezoelectric coefficient d₃₃ of 28 pC/N. This value is the highest value among the modified NLBT-based piezoelectric ceramics examined. The temperature-dependent dielectric spectra show that the Curie temperature T_c of the manganese-modified NLBT ceramics is slightly higher than that of the pure NLBT ceramics. Thermal annealing analysis revealed that the manganese-modified NLBT ceramics possess good thermal stabilities up to 500 °C. These results demonstrate that the manganese-modified NLBT ceramics are promising materials for high temperature piezoelectric applications.     

Mechanism of significantly enhanced piezoelectric performance and stability in textured potassium-sodium niobate piezoelectric ceramics

In this study, the piezoelectric coefficient d₃₃ and planar electromechanical coupling coefficient k_p were enhanced 145% and 71%, respectively for the <001>-textured (K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃ piezoelectric ceramics compared with their randomly oriented counterpart. Significantly enhanced piezoelectric response in textured ceramics is originated from a combined effect of the intrinsic high piezoelectric activity of <001>-oriented grains in the tetragonal-orthorhombic phases, and easy polarization rotation of fine domains. Furthermore, a comparative analysis suggests that <001>-textured ceramics exhibit good thermal stability, benefiting from the weakened depolarization behavior via crystal orientation. The superior fatigue resistance in textured ceramics can be attributed to the reduced clamping effect as low defect density. These results show that high-performance textured ceramics reported in this work will be promising candidates in the field of lead-free piezoelectric materials.     

Enhanced pyroelectric and piezoelectric responses in W/Mn-codoped Bi4Ti3O12 Aurivillius ceramics

The pyroelectric and piezoelectric properties of 4 at% Mn-doped Bi₄Ti_2.9W_0.1O₁₂ (BiTW-Mn) Aurivillius ceramic were investigated and compared to Bi₄Ti_2.9W_0.1O₁₂ (BiTW) counterpart, which were fabricated using a conventional solid state reaction method. High resistivities of 4.9?×?10¹² and 2.5?×?10¹¹ Ω?cm at 100?°C were obtained in the W-doped and W/Mn-codoped BiT ceramics, respectively. They showed similar activation energies and ionic-p-type mixed conduction mechanisms. Higher pyroelectric coefficients of 57.1?μC/m²K and piezoelectric coefficients of 21 pC/N, as well as much lower dielectric loss of 0.003 were achieved in W/Mn-codoped ceramics. These property changes were mainly induced by $\text{M}{\text{n}}_{\text{Ti}}^{}?{\text{V}}_{\text{o}}^{}$ defect dipoles. The effect of acceptor doping was evidenced by an internal bias field, shown by a horizontal offset in the polarization-field behavior. The improved properties together with high thermal stability indicate that BiTW-Mn may be a promising candidate for pyroelectric and piezoelectric devices at elevated temperatures.     

Structural and electrical properties of La3+-doped Na0.5Bi4.5Ti4O15-Bi4Ti3O12 inter-growth high temperature piezoceramics

New lead-free inter-growth piezoelectric ceramics, Na_0.5Bi_8.5-xLa_xTi₇O₂₇ (NBT-BIT-xLa, 0.00≤x≤1.00), were prepared by the conventional solid-state method. Structural and electrical properties of NBT-BIT-xLa were studied. All the NBT-BIT-xLa samples exhibited a single inter-growth structured phase. XRD and Raman spectroscopy revealed a reduced orthorhombicity, which strongly supports the variation of dielectric and ferroelectric properties. Plate-like grains were found to decrease with the increasing x contents. Impedance spectra analysis indicated that oxygen vacancy defects dominated the contributions to the electrical conductivity. The increased activation energies for dc conductivity evidenced the reduction of oxygen vacancy concentration after La substitution, inducing the enhancement in piezoelectric constant (d₃₃) and remanent polarization (2P_r). The studies of thermal depoling indicated that the optimal d₃₃ of NBT-BIT-0.50La ceramics still remained 22 pC/N at 500 °C, implying that this ceramics could be potentially applied into high temperature devices.     

Luminescence and electrical properties of Eu-modified Bi0.5Na0.5TiO3 multifunctional ceramics

The Eu³⁺-modified Bi_0.5Na_0.5TiO₃ (BNT) ceramics have been fabricated by the solid-state reaction method. The impact of Eu³⁺ doping on the structure, photoluminescence, and electrical properties has been studied by XRD, SEM, PL spectra, and LCR meter. X-ray diffraction analysis reveals that the crystal structure of the samples is well matched with the trigonal perovskite, and the optimal temperature of presintering is 880°C. The Eu³⁺-doped BNT ceramics show excellent red fluorescence at 614 nm corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ under 466 nm excitation and relatively long fluorescence lifetime. The BNT-0.02Eu ceramic density is up to 5.68 g/cm³ and the relative density is up to 94.6% with sintering temperature 1075°C. The piezoelectric constant (d₃₃) of samples has been significantly improved up to 110 pC/N by Eu³⁺ doping. The BNT-0.03Eu ceramic presintered at 880°C and sintered at 1050°C has good dielectric properties and excellent luminescence properties. Eu³⁺-doped BNT ceramics make it potential applications for novel integrated electro-optical and multifunctional devices.     

Cold sintering of the ceramic potassium sodium niobate, (K0.5Na0.5)NbO3, and influences on piezoelectric properties

Cold Sintering was applied to densify a Potassium-Sodium Niobate solid solution composition, 0.5KNbO₃-0.5NaNbO₃ (KNN); the process uses a transient chemical sintering aid, moderate pressure (400 MPa), and temperatures between 230–300 °C to obtain ceramics of ～92 to 96 % theoretical density. Typically, sintering temperatures between ～1000?1050 °C are required to density KNN using conventional methods. In this paper, the densification was investigated during heating, particularly the shrinkage in the first 60 min of the cold sintering process. The low-field dielectric and electrical properties of the resulting ceramics were found to be comparable to conventionally sintered KNN. Electric fields up to 80 kV/cm could be applied, however the ceramics showed pinched hysteresis loops, even after poling over a wide range of temperatures and electric fields. A Rayleigh analysis was used to investigate domain dynamics and high reversible permittivity. The irreversible behavior was an order of magnitude lower than in conventionally sintered KNN, likely associated with defect pinning of ferroelectric domains. A Transmission Electron Microscopy (TEM) study revealed a high density of line defects in most grains; dislocations in the grains limit poling and domain wall movement, thus suppressing both the piezoelectric properties and the hysteresis. Furthermore, TEM observations indicated crystalline grain boundaries that were faceted with terrace kink ledges. These observations point to the importance of the initial powder optimization and grain boundary diffusion when using cold sintering to prepare ceramics that are intended to show bulk cooperative properties such as ferroelectricity. The impact of limited high temperature homogenization of bulk diffusional processes is discussed.     

Phase transition and electrical properties of Bi0.5(Na0.8K0.2)0.5ZrO3 modified (K0.52Na0.48)(Nb0.95Sb0.05)O3 lead-free piezoelectric ceramics

In this work, (1−x)(K_0.52Na_0.48)Nb_0.95Sb_0.05O₃−xBi_0.5(Na_0.8K_0.2)_0.5ZrO₃ [abbreviated as (1−x)KNNS−xBNKZ, x=0–0.06] lead-free ceramics were fabricated using solid-state reaction method. The effects of BNKZ contents on the phase structure, piezoelectric and ferroelectric properties were investigated. The phase boundaries including orthorhombic-tetragonal (O-T) and rhombohedral-tetragonal (R-T) multiphase coexistence were identified by XRD patterns and temperature-dependent dielectric constant by adding different content of BNKZ. A giant field induced strain (~0.25%) along with converse piezoelectric coefficient d₃₃^* (~629.4 pm/V) and enhanced ferroelectricity P_r (~38 μC/cm²) were obtained when x=0.02, while the specimen with x=0.03 presented the optimal piezoelectric coefficient d₃₃ of 215 pC/N, due to the O-T or R-T phase coexistence near room temperature respectively. These results show that the introduction of Bi_0.5(Na_0.8K_0.2)_0.5ZrO₃ is a very effective way to improve the electrical properties of (K_0.52Na_0.48)(Nb_0.95Sb_0.05)O₃ lead-free piezoelectric ceramics.     

Electric-field-temperature phase diagram and electromechanical properties in lead-free (Na0.5Bi0.5)TiO3-based incipient piezoelectric ceramics

In this work, the (1-x)(0.8Na_0.5Bi_0.5TiO₃-0.2K_0.5Bi_0.5TiO₃)-xSrTiO₃ (NKBT-xST) incipient piezoelectric ceramics with x = 0–0.07 (0ST-7ST) were prepared by the solid-state reaction method and their structural transformation and electromechanical properties were investigated as a function of ST content. As the ST content increases, the long-range ferroelectric order is disrupted, and the ferroelectric-relaxor phase transition temperature (T_FR) shifts to around room temperature for NKBT-5ST ceramics, accompanied by a relatively high electrostrain of 0.3% at 6 kV/mm. The large strain response associated with the vanished ferroelectric properties around T_FR can be attributed to the reversible relaxor-ferroelectric phase transition. The electric-field-temperature (E-T) phase diagrams were established, and the transition between the two field-induced long-range ferroelectric states were found to take place via a two-step switching process through an intermediate relaxor state. The threshold electric field to trigger the conversion between ferroelectric state and relaxor state depends strongly on the dynamics of polarization relaxation, which is influenced by temperature and composition.     

Large enhancement of piezoelectric properties and resistivity in Cu/Ta co-doped Bi4Ti3O12 high-temperature piezoceramics

In this study, the electrical properties of Bi₄Ti₃O₁₂-based Aurivillius-type ceramics were tailored by a B-site co-doping strategy combining high valence Ta⁵⁺ and low valence Cu²⁺. A series of Bi₄Ti_3−x(Cu_1/3Ta_2/3)_xO₁₂ (BTCT) (x = 0, 0.005, 0.01, 0.015, 0.02, 0.025, and 0.03) ceramics were prepared by the conventional solid-state reaction method. The effect of Cu/Ta co-doping on the crystal structure, microstructure, dielectric properties, piezoelectric properties, ferroelectric properties, and electrical conductivity of these ceramics was systematically investigated. Co-doping significantly enhanced the piezoelectric properties and DC electrical resistivity of the resulting composites. The optimized comprehensive performances were obtained at x = 0.015 with a large piezoelectric coefficient (34 pC/N) and a relatively high resistivity of 9.02 × 10⁶ Ω cm at 500°C. Furthermore, the ceramic also exhibited stable thermal annealing behaviors and excellent fatigue resistance. The results of this study demonstrated great potential of the Cu/Ta co-doped Bi₄Ti₃O₁₂ ceramics for high-temperature piezoelectric device applications.     

Enhanced piezoelectric property and promoted depolarization temperature in Fe doped Bi1/2(Na0.8K0.2)1/2TiO3 lead-free ceramics

Piezoelectric sensors and energy harvesters require piezoelectric materials with large piezoelectric responses and good thermal stability. However, a commonly accepted concept is that the promotion of depolarization temperature of Bi_1/2Na_1/2TiO₃-based lead-free ceramics is usually companied by deterioration of piezoelectric properties. In the present study, the effects of acceptor-Fe doping on piezoelectric property and thermal depolarization behavior of Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ ceramics are investigated. Fe doping at an appropriate level (≤ 3.0%) improves piezoelectric property and thermal stability simultaneously, due to the stabilization of long-range ferroelectric order. Piezoelectric constant d₃₃ increases from 125 pC/N to 148 pC/N with Fe amount of 3.0%, and then decreases. The depolarization temperature T_d is promoted continuously with Fe addition, from 76 °C for the undoped sample to 118 °C for the sample with Fe amount of 5.0%. It is proposed that the piezoelectric property and thermal stability can be simultaneously improved by stabilizing the long-range ferroelectric order in Bi_1/2Na_1/2TiO₃-based systems with obvious relaxor character. This work provides a new insight into the improvement of Bi_1/2Na_1/2TiO₃-based lead-free piezoelectric ceramics.     

Processing and enhanced electrical properties of Sr1-x(K0.5Bi0.5)xBi2Nb2O9 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, Sr_1−x(K_0.5Bi_0.5)_xBi₂Nb₂O₉ (SKBN-x, x=0, 0.2, 0.5, 1.0), were synthesized by a conventional solid-state reaction. Structural and electrical properties of SKBN-x ceramics were investigated. X-ray diffraction analysis suggested that the substitution led to the formation of a layered perovskite structure. Plate-like morphologies for the grains were clearly observed in all the samples, which are characteristic for layer-structure Aurivillius compounds. The Curie temperature (T_c) is found to shift to higher temperature from 445 °C to 509 °C with increasing (K, Bi) content. Excellent remanent polarization (2P_r∼15 μC/cm²) were obtained for SKBN-0.2 ceramic. High piezoelectric coefficient of d₃₃∼21  pC/N were obtained for the samples at x=0.5. Additionally, thermal annealing studies indicated that the piezoelectric coefficient (d₃₃) of SKBN-0.5 was unchanged even if annealing temperature increased to be 450 °C, demonstrating the ceramics are the promising candidates for high-temperature applications.     

Enhancement of piezoelectric and ferroelectric properties of BaTiO3 ceramics by aluminum doping

Ferroelectric and piezoelectric properties of BaTiO₃ and Al-doped BaTiO₃ ceramics were investigated. The ferroelectric study demonstrated that, by doping Al³⁺ ions in the A-site of BaTiO₃, the polarization–electric field loop exhibited enhanced remnant polarization (from 12 to 17.5  μC/cm²), saturation and switching. In addition, the piezoelectric constant (d₃₃) increased with Al-doping for both static and dynamic strain values (from 75 to 135 and from 29.2 to 57.9 pC/N, respectively, at a maximum applied electric field of 16 kV/cm). Furthermore, the dielectric constant values increased and both the dielectric loss factor and leakage current decreased, even though the transition temperature shifted to lower temperature (from 121 to 113 °C) for the Al-doped sample. Therefore, the Al-doped BaTiO₃ has adjustable piezoelectric and ferroelectric properties.     

Effects of Fe2O3 doping on the electrical properties of Na0.47Bi0.47Ba0.06TiO3 lead-free ceramics

The Na_0.47Bi_0.47Ba_0.06Ti_1-xFe_xO_3-Δ lead-free piezoelectric ceramics (BNBT-100xFe, x?=?0, 0.01, 0.02, 0.03) were synthesized by using the solid-state reaction technique. X-ray powder diffraction patterns demonstrate that the doping Fe₂O₃ has totally diffused into the crystal lattice of the ceramics and form a pure perovskite structure. Enhanced piezoelectric property is obtained at x?=?0.01, which is reflected on the enhanced remnant polarization (P_r) and a giant piezoelectric constant (d₃₃) up to 168 pC/N. The increasing ferroelectric-to-relaxor phase transition temperature (T_F-R) on dielectric permittivity curves suggest the enhanced ferroelectric characteristics with increasing the Fe³⁺ content. By using the complex ac impedance analysis, the grain, grain boundary and electrode effects are all detected at the appreciate composition. The resistivity behavior of the samples is sensitive to the doping Fe³⁺ concentration, and additionally, the oxygen vacancies play an important role in this characteristic.     

Enhancement of pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics by La doping

Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La³⁺ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (T_d) decreases. p is 7.24 × 10⁻⁴C m⁻² °C⁻¹ at RT at x = 0.5% and 105.4 × 10⁻⁴C.m^−2 °C⁻¹ at T_d at x = 0.2%. F_i and F_v show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 ⁻¹⁰ m v⁻¹) (x = 0.5%) and from 0.021 to 0.048 (m².C⁻¹) respectively. F_i and F_v show a huge increase to 37.6 × 10⁻¹⁰ m v⁻¹ and 0.56 m² C⁻¹ respectively at T_d at x = 0.2%. F_C shows values of 2.10, 2.89, and 2.98 (x10⁻⁹C cm⁻² °C⁻¹) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications.     

Room temperature constructing rhombohedral-tetragonal phase boundary in novel (Bi,Na)(Zr,Ti)O3 modified (K,Na)(Nb,Sb)O3 ceramics: Phase structure,defect and piezoelectric performance

Lead-free (1-x)(K_0.5Na_0.5)(Nb_0.96Sb_0.04)O₃-x(Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃ ceramics (abbreviated as (1-x)KNNS-xBNZT, x = 0, 0.01, 0.02, 0.03, 0.035 and 0.04) were synthesized by the solid-state method, and the dependence of phase evolution, microstructure, oxygen vacancy defect and electrical properties on compositions were carefully investigated. All ceramics had a pure perovskite structure and a dense microstructure. The phase transition temperatures (T_R-O and T_O-T) of the ceramics were adjusted by adding BNZT, and the rhombohedral-tetragonal (R-T) phase coexistence boundary was successfully constructed at room temperature when x = 0.03, the excellent piezoelectric performance (d₃₃ ～ 323 pC/N, k_p ～ 0.372) and high Curie temperature (T_C ～ 276 °C) have been achieved at this time. The grain size of the ceramics showed a strong difference on x content, and the maximum relative density value of 95.42% was obtained. The domain structure characterized by PFM confirmed that the ceramics possess small-sized nano-domains and complex domains at x = 0.03, which are the origin of enhanced piezoelectric properties. Moreover, the oxygen vacancy defect that can pin the domain walls was increased with the addition of (Bi_0.5Na_0.5)(Zr_0.8Ti_0.2)O₃. As a result, the doping with BNZT can significantly affect the phase structure and electrical properties of the ceramics, indicating that the (1-x)KNNS-xBNZT ceramics system with a R-T phase boundary is a promising lead-free piezoelectric material.