Tetragonal tungsten bronze phase potential in increasing the piezoelectricity of sol-gel synthesized (K0.5Na0.5)1-xLixNbO3 ceramics

(K,Na)NbO₃ (KNN)-based ceramics have been proven to be formidable candidates among lead-free piezoelectric materials, yet poor reproducibility always hinders their progress. In the present study, the effects of low lithium substitution on the electrical properties and microstructure of (K_0.5Na_0.5)_1-xLi_xNbO₃ (KNLN) ceramics were investigated. All samples were synthesized by the sol-gel method. The Curie temperature (T_C) of the ceramics shifted to higher temperature and gradually decreased the monoclinic-tetragonal (T_M-T) phase transition. Li⁺ substitution had a prominent effect on the ferroelectric properties and improved the piezoelectric coefficient (d₃₃) up to 181 pC/N. X-Ray Diffraction (XRD) studies and Field Emission Scanning Electron Microscopy (FESEM) images revealed an inevitable tetragonal tungsten bronze (TTB) secondary phase, which was formed during the preparation process. It was demonstrated that the volatilization of Li⁺ cations facilitated TTB growth. The coexistence of two different phase structures proved to enhance the KNN piezoelectric performance.     

Structural and electrical properties of Na0.47K0.47Li0.06NbO3 lead-free piezoelectric ceramics modified by AgSbO3

(1?x)Na_0.47K_0.47Li_0.06NbO₃ (NKLN)–xAgSbO₃ lead-free piezoelectric ceramics were prepared using a reaction sintering method. The effects of AgSbO₃ doping on the structural and electrical properties of NKLN ceramics sintered at 1000–1040 °C were studied. The dopant affected densification, phase content, sintering temperature, microstructure and electrical properties. Variations in the relative intensity of X-ray diffraction peaks were consistent with Ag⁺ and Sb⁵⁺ ions substituting on the perovskite lattice to produce a change in the proportions of co-existing tetragonal and orthorhombic phases. Grain growth during secondary re-crystallization was also affected. The temperature of the orthorhombic–tetragonal (O–T) phase transition and the Curie temperature (T_C) decreased as a result of AgSbO₃ modifications. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic–tetragonal polymorphotropic phase boundary. The 0.92Na_0.47K_0.47Li_0.06NbO₃–0.08AgSbO₃ ceramics exhibited optimum electrical properties (d₃₃=252 pC/N, ε_r=1450, tan δ=0.02, and T_C=280 °C). These results reveal that (1?x)Na_0.47K_0.47Li_0.06NbO₃–xAgSbO₃ ceramics are promising materials for lead-free piezoelectric application.     

Low-Temperature Sintering of (K0.5Na0.5)NbO3 Piezoelectric Ceramics

(K_0.5Na_0.5)NbO₃ piezoelectric ceramics can be sintered at a temperature as low as 750 °C for 5 h by incorporating Li₂CO₃ + Bi₂O₃ + ZnO as the sintering aid, whereas the conventional sintering temperature is around 1,100 °C. The optimal “soft” piezoelectric properties are obtained for ceramics sintered at 850 °C for 5 h. The dielectric permittivity (ε), piezoelectric coefficient (d ₃₃), electromechanical coupling (k _p) and mechanical quality factors (Q _m) of (K, Na)NbO₃ modified with 5.5 wt% sintering aids are 1,436, 90 pC/N, 0.3 and 10, respectively. These values are similar to the values obtained for (K_0.5Na_0.5)NbO₃ ceramics sintered above 1,100 °C. The underlying mechanism for abrupt change of dielectric permittivity is explained.     

The effect of B-site substitution on the structural evolution and electrical properties of lead-free (K,Na)NbO3 ceramics

Lead-free (K_0.49Na_0.51)(Nb_1−xSb_x)O₃ piezoelectric ceramics were prepared via the conventional sintering method and the effect of the substitution of Nb with Sb on the phase structure, microstructure and electrical properties of the prepared (K_0.49Na_0.51)(Nb_1−xSb_x)O₃ ceramics was systematically investigated. The prepared ceramics exhibited a single-phase perovskite structure which changed from a standard orthorhombic structure to a pseudocubic structure as x was increased from 0 to 0.1. X-ray diffraction patterns and Raman spectra obtained for the prepared ceramics clearly revealed that the degree of structural symmetry increased with x. Substituting an appropriate amount of Sb⁵⁺ for Nb⁵⁺ was found to improve the microstructure and thereby enhance the piezoelectric/ferroelectric properties. Further increasing the Sb content resulted in a decrease of the average grain size and a deterioration of the performance. The peak values of the piezoelectric constant d₃₃ (182 pC/N) and the electromechanical coupling coefficient k_p (41%) were obtained for the ceramic with x=0.06.     

Temperature stability and phase transition of Li0.02(KxNa1−x)0.98NbO3 ceramics

Li_0.02(K_xNa_1?x)_0.98NbO₃(x = 0.35–0.55) ceramics were prepared using the conventional solid state sintering method. The thermal behaviors of Li-modified (K_xNa_1?x)NbO₃ ceramics were investigated from ?30 to 150 °C, and the effect of Na/K ratio in (K_xNa_1?x)NbO₃ ceramics on thermal behavior and electrical properties was also studied. In the case of Li_0.02(K_xNa_1?x)_0.98NbO₃ ceramics with 0.5 wt.% ZnO, the transition temperature was sharply decreased because of a phase transition as the composition range of x was 0.425–0.475. From the results of the temperature dependence of piezoelectric properties, it is assumed that the Na-rich phase is less stable than the K-rich phase for temperature change.     

Effect of SrZrO3 on phase structure and electrical properties of 0.974(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3 lead-free ceramics

(0.974−x)(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–xSrZrO₃ lead-free piezoelectric ceramics have been prepared by the conventional solid state sintering method. Systematic investigation on the microstructure, crystalline structures as well as electrical properties of the ceramics was carried out. With the addition of SrZrO₃, the rhombohedral–orthorhombic phase transition temperature of the ceramics increases. Both the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions of the ceramics were modified to be around room temperature when x~0.05, and as a result remarkably strong piezoelectricity has been obtained in 0.924(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–0.05SrZrO₃ ternary system, whose piezoelectric parameters were d₃₃=324 pC/N and k_p=41%.     

Piezoelectric enhancements in K0.5Na0.5NbO3-based ceramics via structural evolutions

The current work aims to compare the effect of systematic A-site and B-site substitutions on the piezoelectricity of Ka_0.5Na_0.5NbO₃ (KNN)-based perovskite ceramics. The A-site elements was replaced by Li⁺ while Nb5⁺ was substituted by Sb⁵⁺ to form (K_0.4675Na_0.4675Li_0.065)NbO₃ (KNLN) and (K_0.4675Na_0.4675Li_0.065)(Nb_0.96Sb_0.04)O₃ (KNLNS) respectively. The ceramics were prepared using solid-state sintering method. The density of the ceramics steadily improved with the substitutions while the crystal structure evolved from monoclinic (in KNN) to the coexistence of monoclinic and tetragonal (in KNLN) and finally tetragonal in KNLNS. Distinct variations on size and morphology were recorded. Although density, crystal structure and morphology have minor effect on the E_c, they imposed considerable influences on P_r, d₃₃ and k_p. Despite relatively lower density, KNLN exhibited the highest P_r, d₃₃ and k_p at 9.80 μC/cm²,185 pC/N and 0.43 respectively signifying the positive enhancement brought by the co-existence of monoclinic and tetragonal crystal structures. More importantly, this work systematically proved that the co-existence of both structures signified the morphotropic phase boundary (MPB) composition as the primary factor for the enhancement of KNN piezoelectric properties.     

Effects of Sb content on electrical properties of lead-free piezoelectric (K0.4425Na0.52Li0.0375) (Nb0.9625−xSbxTa0.0375)O3 ceramics

Lead-free (K_0.4425Na_0.52Li_0.0375) (Nb_0.9625−xSb_xTa_0.0375)O₃ piezoelectric ceramics were prepared by the conventional sintering method. The effects of the Sb content on the phase structure, microstructure, dielectric, piezoelectric, and ferroelectric properties of the (K_0.4425Na_0.52Li_0.0375) (Nb_0.9625−xSb_xTa_0.0375)O₃ ceramics were investigated. The much higher Pauling electronegativity of Sb compared with Nb makes the ceramics more covalent. By increasing x from 0.05 to 0.09, all samples exhibit a single perovskite structure with an orthorhombic phase over the whole compositional range, and the bands in the Raman scattering spectra shifted to lower frequency numbers. The grain growth of the ceramics was improved by substituting Sb⁵⁺ for Nb⁵⁺. Significantly, the (K_0.4425Na_0.52Li_0.0375) (Nb_0.8925Sb_0.07Ta_0.0375)O₃ ceramics show the peak values of the piezoelectric coefficient (d₃₃), electromechanical coupling coefficient (k_p), and dielectric constant (?), which are 304 pC/N, 48% and 1909, respectively, owing to the densest microstructure of typical bimodal grain size distributions. Besides, the underlying mechanism for variations of the electrical properties due to Sb⁵⁺ substitution was explained in this work.     

Domain structure of nonstoichiometric sodium potassium niobate-based ceramics for piezoelectric acoustic actuators

In this work, three piezoelectric acoustic actuators are prepared by using multi-layer technology for lead-free (Na_0.48-xK_0.48-xLi_0.04)Nb_0.89-xTa_0.05Sb_0.06O₃-xSrTiO₃ (NKLNTS-xST) ceramics, x=0 and 0.007, and commercial lead-based ceramic. The phase and domain structure are also investigated by using the XRD patterns and TEM images of the NKLNTS-xST ceramics. The (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ piezoelectric acoustic actuator has the best sound pressure level and d₃₃ value of 650 pCN^?1. The response mechanisms suggest that the piezoelectric ceramic vibration amplitude was increased and the sound pressure level improved since the orthorhombic and tetragonal phases were found to coexist and the nanoscale domain increased for the (Na_0.473K_0.473Li_0.04Sr_0.007)Nb_0.883Ti_0.007Ta_0.05Sb_0.06O₃ ceramic.     

Optical and electrical properties of pressureless sintered transparent (K0.37Na0.63)NbO3-based ceramics

Lead-free (1−x)(K_0.37Na_0.63)NbO₃-xCa(Sc_0.5Nb_0.5)O₃ (x=0.050, 0.070, 0.090, 0.095 and 0.100) transparent ferroelectric ceramics have been fabricated by pressureless sintering procedure. Transmittance of 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics sintered in sealed alumina crucible was 15% higher than those sintered unsealed in air. By increasing the content of Ca(Sc_0.5Nb_0.5)O₃, the phase structure of (K_0.37Na_0.63)NbO₃ ceramics transformed from orthorhombic to tetragonal symmetry first and then to pseudo cubic symmetry. The 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ ceramics exhibited high density (98%), high transmittance (60%) in the near-IR region and relatively good electrical properties (ε_r=1914, tanδ=0.037, T_c=147 °C, P_r=6.88 μC/cm², E_c=8.49 kV/cm). Meanwhile, the introduction of Ca(Sc_0.5Nb_0.5)O₃ induced a composition fluctuation in the (K_0.37Na_0.63)NbO₃ lattice and made the ceramics more relaxor-like, which would lead to a further reduction of light scattering. These results demonstrated that 0.91(K_0.37Na_0.63)NbO₃-0.09Ca(Sc_0.5Nb_0.5)O₃ could be promising lead-free transparent ferroelectric ceramics.     

Ferroelectric‒to‒relaxor crossover in KNN‒based lead‒free piezoceramics

This study investigated the phase transition behavior and electrical properties of (K_0.5Na_0.5)(Nb_1-xZr_x)O₃ (KNN?100xZ) and (K_0.5Na_0.5)NbO₃–yBaZrO₃ (KNN–100yBZ) lead–free piezoelectric ceramics. The phase transitions in crystal structures were compared in KNN ceramics between single Zr⁴⁺ doping and Ba²⁺Zr⁴⁺ co?doping. Piezoelectric properties such as the piezoelectric constant (d₃₃) and electromechanical coupling factor (k_p) are optimized for KNN?6BZ ceramics and were clarified via the polymorphic phase transition from the orthorhombic to pseudocubic phase. The fitted degree of diffuseness (γ) for a phase transition from the modified Curie–Weiss law indicated that KNN ceramics as ferroelectrics are gradually transformed through BaZrO₃ modification. Accordingly, the enhanced strain properties at y = 0.08 consist of coexisting ferroelectric domains and polar nanoregions that are supported by ferroelectric–to–relaxor crossover in KNN?100BZ ceramics.     

Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics

In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K_0.48Na_0.52)(Nb_1-ySb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (abbreviated as (1-x)KNN_1-yS_y-xBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d₃₃¹ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.     

Ultrahigh electrostrictive effect in potassium sodium niobate-based lead-free ceramics

The longitudinal electrostrictive coefficient Q₃₃ for perovskite-structured ferroelectric ceramics is usually between 0.01?0.04 m⁴/C². However, an ultra-low Q₃₃ of only 0.0047 m⁴/C² was identified in the 0.9K_0.5Na_0.5NbO₃-0.1SrTiO₃ (KNN-ST) composition. Despite the fact that superior piezoelectricity has been observed in KNN-based ceramics, this value is obviously much smaller than the normal value, according to the general cognition and the thermodynamic relationship between piezoelectric coefficient d₃₃ and Q₃₃. Therefore, we synthesized (1?x)(K_0.45Na_0.49Li_0.06)NbO₃-xSrTiO₃ (KNLN-ST) and studied phase structure, dielectric and ferroelectric properties systematically. Our findings show that the Q₃₃ in the KNLN-ST system (0.012?0.027 m⁴/C²) is within the reasonable range for perovskite-structured ferroelectric ceramics. Furthermore, an ultra-high electrostrictive strain (>0.3%) with ultra-low hysteresis was achieved in the 0.8KNLN-0.2ST sample. This research not only clarifies the electrostrictive effect in KNN-based systems, but it also broadens the potential application field of KNN-based ceramics to electrostrictive actuators.     

Photoluminescence,thermoluminescence and reversible photoluminescence modulation of multifunctional optical materials Pr3+ doped KxNa1-xNbO3 ferroelectric ceramics

It is highly significant to develop multifunctional optical materials to meet the huge demand of modern optics. Usually, it is difficult to realize multiple optical properties in one single material. In this study, we choose ferroelectric (K_xNa_1-x)NbO₃:Pr³⁺ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) as hosts, and the rare earth ions Pr³⁺ are doped in them. For the first time, the integration of photoluminescence, photochromism, luminescence modulation and thermoluminescence and has been achieved in the Pr³⁺ doped (K_xNa_1-x)NbO₃:Pr³⁺ ferroelectric ceramics. Upon 337- or 448-nm light irradiation, all samples show strong red emissions centered at 610 nm. The photochromic reaction increases with the increasing K⁺ content in the (K_xNa_1-x)NbO₃:Pr³⁺ ceramics. A strong photochromic reaction has been found in the (K_0.5Na_0.5)NbO₃:Pr³⁺ ceramics. Accordingly, a large and reversible photoluminescence modulation (ΔR_t = 50.71%) is achieved via altering 395-nm-light irradiation and 200 °C thermal stimulus. All the prepared ceramics show a visible thermoluminescence when stimulated at 200 °C. The mechanisms of luminescence modulation and thermoluminescence are discussed. Present study could provide a feasible paradigm to realize multiple optical properties in one single material.     

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.     

Phase structure and enhanced piezoelectric properties in (1-x)(K0.48Na0.52)(Nb0.95Sb0.05)O3-x(Bi0.5Na0.42Li0.08)0.9Sr0.1ZrO3 lead-free piezoelectric ceramics

The piezoelectric properties of KNN lead-free piezoelectric ceramics could be greatly enhanced by forming multiphase coexistence. In this work, binary system (1-x)(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-x(Bi_0.5Na_0.42Li_0.08)_0.9Sr_0.1ZrO₃ [(abbreviated as (1-x)KNNS-xBNLSZ] ceramics with rhombohedral-tetragonal (r-T) phase boundary was designed and synthesized using the conventional solid-state sintering method, and effects of BNLSZ contents on their micrograph, phase structure and electrical properties were also investigated. According to phase diagram from the results of temperature-dependent capacitance and dielectric constant, the ceramics exhibit the R-T phase coexistence in the composition range of 3.5%≤x<4.5%, and an enhanced dielectric, ferroelectric, and piezoelectric behavior was obtained at such a phase boundary zone. As a result, the ceramics with x=0.04 exhibit optimum electrical properties of d₃₃~461 pC/N, k_p~46%, tan δ~0.03, P_r~16.9 μC/cm², and E_c ~9 kV/cm, together with a Curie temperature (T_C) of ~228 °C. Such a good comprehensive performance obtained in this present work is due to the R-T phase transition and enhanced ɛ_rP_r. It was believed that this ceramic system would promote the development of KNN-based lead-free ceramics.     

Understanding the origin of the high piezoelectric performance of KNN-based ceramics from the perspective of lattice distortion

Although both phase structure and fraction can affect the electrical properties of (K, Na)NbO₃ based ceramics, few studies have examined the effects of lattice distortion on their electrical properties. In this work, we prepared (1-x)K_0.48Na_0.52NbO₃-x(NaSbO₃-(Bi_0.45Sm_0.05)Li_0.5ZrO₃) lead free piezoelectric ceramics with a multiphase coexistence region by regulating the doping component. The c/a ratio of the tetragonal phase greatly influenced the electric properties of the ceramics. The c/a ratio of the tetragonal phase decreases from 1.0102 to 1.0080 and subsequently increases to 1.0090. When the c/a of the tetragonal phase reaches its minimum value, the ceramic with x = 0.04 has the best piezoelectricity (d₃₃～348 pC/N). Our research exhibits an alternative way to further understand the mechanism of electric property improvement of (K, Na)NbO₃ based ceramics with multiphase coexistence.     

A study on epoxy‐based 1–3 piezoelectric composites using finite element method

The article deals with finite element study of 1–3 composites made of piezoceramic fibers embedded in epoxy. The study focuses on evaluation of effective properties of piezoelectric fiber composites using representative volume element method. The physical properties deduced are further used for analyzing sensing and actuation characteristics of unimorph cantilever beam. K_0.5Na_0.5NbO₃‐LiSbO₃ (KNN‐LS), KNN‐LS doped with 1 wt% CaTiO₃ [KNN‐LS‐CT (1 wt%)], K_0.475Na_0.475Li_0.05(Nb_0.92Ta_0.05Sb_0.03)O₃ (KNLNTS), 0.885 (Bi_0.5Na_0.5)TiO₃−0.05(Bi_0.5K_0.5)TiO₃−0.015 (Bi_0.5Li_0.5) TiO₃ −0.05 BaTiO₃ (BNKLBT) and lead zirconate titanate (Pb [Zr_xTi_1−x] O₃) (PZT) are embedded in epoxy matrix to form 1–3 piezocomposites. K_0.475Na_0.475Li_0.05(Nb_0.92Ta_0.05 Sb_0.03)O₃ (KNLNTS) shows excellent performance than other composites under study. It can be concluded that KNLNTS (lead‐free piezoelectric material) is a potential candidate which can replace lead‐based PZT for smart structure applications. POLYM. COMPOS., 37:1895–1905, 2016. © 2015 Society of Plastics Engineers     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.     

Dielectric,ferroelectric, and piezoelectric properties in potassium sodium niobate ceramics with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries

A new phase boundary with rhombohedral–orthorhombic and orthorhombic–tetragonal phase boundaries is designed in (K_0.48Na_0.52)NbO₃ by adding Bi_0.5(Na_0.7K_0.2Li_0.1)_0.5ZrO₃ (BNKLZ), where Zr⁴⁺ and (BNKL)²⁺ are respectively used to improve the temperature of a rhombohedral phase and to decrease the temperature of an orthorhombic–tetragonal phase coexistence. These ceramics endure several continuous phase transitions with increasing BNKLZ content, i.e., an orthorhombic phase (0≤x<0.03), orthorhombic–tetragonal phases (x=0.03), orthorhombic–tetragonal and rhombohedral–orthorhombic (O–T and R–O) phase existence (0.03<x≤0.05), a rhombohedral phase (0.05<x≤0.07). The ceramics with O–T and R–O have a better piezoelectric behavior as compared with other phases because of more polarization states, enhanced ε_r and P_r, and a dense microstructure. Moreover, piezoelectric properties could be further optimized by modifying their sintering and poling temperatures. As a result, the construction of O–T and R–O phase coexistence benefits the improvement of piezoelectric properties in KNN-based ceramics.