Beyond phase boundaries: atomic mechanisms governing structure and property variations in (K, Na)NbO_3-based ferroelectrics

Domain: physics
Published: 2025-12-13
Editorial outcome: major_revision

Abstract
Chemical dopants-induced phase boundary engineering has boosted electrical properties of (K, Na)NbO3-based piezoceramics, yet the underlying mechanisms governing these improvements remain unclear. Here, we elucidate these mechanisms through comprehensive multi-scale structural analysis (atomic-to-nanoscale-to-mesoscale) on two representative solid-solutions, namely (K, Na, Li)NbO3 and (K, Na)NbO3-(Bi0.5Na0.5)ZrO3. By utilizing neutron pair distribution function analysis, scanning transmission electron microscope, first-principle calculations, and phase-field simulations, our results reveal distinct atomic-scale mechanism underlying phase boundary engineering. In (K, Na, Li)NbO3, convergent off-center displacements of Li atoms induce an interplay between displacive and order-disorder phase transition; while in (K, Na)NbO3-(Bi0.5Na0.5)ZrO3, divergent off-center displacements of Bi atoms trigger a predominant order-disorder type phase transition. These atomic-scale structural characteristics directly correlate with mesoscopic ferroelectric domains and ultimately determine macroscopic electrical properties. This work elucidates the role of chemical dopants in phase boundary engineering from a multi-scale perspective, establishing a framework for designing lead-free piezoceramics with enhanced electrical properties and advancing the development of eco-friendly piezoceramics.

Introduction
Piezoceramics, well-known for their ability to interconvert electrical and mechanical energies, serve as indispensable functional components in a variety of electronic devices, such as sensors, actuators, and transducers1. Historically, lead(Pb)-based piezoceramics, predominantly the lead zirconate titanate {Pb(Zr,Ti)O3, PZT} family, have dominated the market2. However, the widespread use of Pb-based commercial piezoceramics raises significant environmental and health concerns due to the toxicity of Pb3. In response to this challenge, substantial efforts have been made to develop eco-friendly Pb-free alternatives, among which potassium sodium niobate {(K,Na)NbO3, KNN} based ceramics stand out due to their excellent piezoelectric properties4.

The documented high piezoelectric properties of KNN-based ceramics are intimately related to the phase boundary engineering (PBE), a strategy that regulates the phase transition temperature, notably the orthorhombic/monoclinic-tetragonal phase transition temperature (TO/M-T) to occur near room temperature5. This modulation is typically achieved through the addition of dopants, such as lithium (Li) or (Bi0.5M0.5)NO3 (M=Na, K; N=Zr, Hf) dopants6. Recent crystallographic studies, including our work, indicate that the monoclinic (M) phase better describes KNN ceramics than the traditional orthorhombic (O) phase7–9, therefore we consistently refer to this structural feature as the M phase hereafter.

Several physical mechanisms have been proposed to interpret the enhanced performance induced by PBE, including the multi-phase coexistence10,11, nano-sized ferroelectric domains12, and electric-induced phase transitions13,14. It is well established that chemical dopants directly influence atomic displacements and octahedral tilting/rotation in individual unit cells, which subsequently induces nano-sized domains or phase transition. However, these proposed mechanisms primarily account for enhancements at the meso-/macroscopic scale, falling to provide comprehensive explanations at the atomic scale. This knowledge gap stems from limited investigations into local atomic structures, particularly evident in KNN-based solid-solutions, exhibiting similar M-T phase boundaries but distinct electrical properties (as exemplified in Fig. 1). Consequently, the development of high-performance KNN-based ceramics remains largely empirical and relies heavily on trial-and-error approaches. Deciphering how different chemical dopants affect local atomic structures is therefore crucial for deeper understanding the fundamental mechanisms of PBE formation and for explaining the observed differences in electrical properties induced by chemical modifications.

In this work, we conduct a comprehensive multi-scale structural analysis, with particular emphasis on local atomic scale, to explain the role of chemical dopants towards construction of the phase boundaries in KNN-based ceramics. Our approach integrates neutron total scattering, scanning transmission electron microscopy (STEM), first-principles calculations, and phase-field simulations, in providing a detailed examination on two representative solid-solution systems: (1−x)(K0.48Na0.52)NbO3−xLiNbO3 and (1−y)(K0.48Na0.52)NbO3−y(Bi0.5Na0.5)ZrO3 (abbreviated as KNN-xLi and KNN-yBNZ, respectively). Through this multi-technique investigation, we elucidate the distinctive role of Li and BNZ in constructing the M-T phase boundary and demonstrate how these specific dopants contribute to the observed variations in electrical properties.

Results
We first compare two KNN-based ceramics with similar M-T phase boundaries but distinct electrical properties: KNN-xLi and KNN-yBNZ systems. The chemical composition analysis reveals the elemental ratio close to the nominal value, and Li occupies the A-site in KNN-xLi ceramics (Figs. S1–S3). X-ray diffraction (XRD) patterns reveal a transition in the long-range crystal symmetry from an M (monoclinic) to T (tetragonal) phase structure in both solid-solution systems with increasing dopant content, evidenced by a clear shift in the intensity ratio between (002) and (200) peaks from 2:1 to 1:2 (Fig. 1a, b and Fig. S4). Temperature-dependent dielectric properties further show a decrease in the TM-T with increasing Li and BNZ content (Fig. 1c–e and Figs. S5–S6). Despite these structural similarities, significant differences emerge in their dielectric behaviors. KNN-0.06Li shows a sharp dielectric anomaly with low permittivity (i.e., ε′ for unpoled or ε330 for poled) of 500–700 at TM-T, whereas KNN-0.06BNZ demonstrates a diffused dielectric anomaly with a higher ε′ (or ε330) of 1200–1500. This distinction extends to domain morphologies as revealed by piezoelectric force microscopy (PFM). While pristine KNN ceramics exhibit regular striped domains averaging ~0.5 μm in width and ~3.5 μm in length (Fig. 1f), KNN-0.06Li ceramics show sub-micron-sized striped domains (e.g., 0.4–0.8 μm in width) overlapped with irregular watermark-shaped domains (Fig. 1g). KNN-0.06BNZ exhibits, however, distinctly irregular and fragmented domains with sizes of 0.3–0.6 μm (Fig. 1h). Additional PFM results and related analysis are shown in Figs. S7–S10.

Ferroelectric hysteresis (P–E) loops and electric-induced strain (S–E) curves further differentiate these two systems. Li doping shows minimal improvement in polarization and strain properties, and causes a decline at higher concentrations (e.g., x ≥ 0.07) (Fig. 1i, j and Fig. S11). Conversely, an optimal BNZ content (e.g., y=0.045) significantly improves the polarization and strain properties, though excessive BNZ deteriorates performance. The piezoelectric coefficient (d33), which is proportional to the product of relative dielectric permittivity (εr) and remanent polarization (Pr), increases by a factor of ~3.6 in KNN-0.06BNZ and ~2 in KNN-0.06Li compared to pristine KNN (Fig. 1k and Fig. S11). Thus, d33 enhancement of both systems depends on εr rather than Pr because they show lower Pr values than that of pristine KNN ceramics.

Several critical questions arise regarding the observed differences in electrical properties and domain morphologies between KNN-0.06Li and KNN-0.06BNZ, despite their similar M-T phase boundaries. First, why does KNN-0.06BNZ exhibit a room-temperature εr approximately twice as high as that of KNN-0.06Li? εr is determined by both intrinsic (e.g., crystal structure) and extrinsic factors (e.g., grain size and domains). Both systems share the similar M-T phase coexistence and comparable domain sizes (Figs. S7 and S8), suggesting these factors alone cannot explain the disparity. Furthermore, the larger grain size of KNN-0.06Li (Fig. S12), which typically enhances εr15, corresponds instead to a lower value, indicating additional mechanisms are involved. Second, why do two systems show different trends of Curie temperature (TC) and diffusion factor varying with composition (Fig. S13)? BNZ doping makes the dielectric anomaly at TM-T dispersive and decreases TC, whereas the opposite situations occur with Li doping. TC is directly proportional to the spontaneous polarization (Ps)16. While Pb-/Bi-doped ceramics achieve higher Ps through strong hybridization of Pb–O and Bi–O bonds17, Li–O bonds exhibit weak hybridization18. Third, why does KNN-0.06BNZ exhibit more fragmented domains compared to KNN-0.06Li? (Figs. S7 and S8). This is likely related to the heterovalent ionic substitutions of BNZ that introduces local heterogeneities in KNN ceramic matrix19. These questions hint at the fact that Li and BNZ additions affect the local atomic structures near the M-T phase boundary in fundamentally different ways. Therefore, we conduct a multi-scale structural analysis, with a particular focus on local atomic structures, to clarify these mechanisms.

To quantitatively characterize the M-T phase transition behavior, Rietveld refinements are performed on neutron Bragg diffraction patterns to analyze structural parameters, including unit cell constants, atomic coordinates, and atomic displacement parameters (ADPs) (Figs. S14–S17)20. These refinements indicate a structural transition from an M phase in KNN to a coexistence of M and T phases in KNN-0.06Li and KNN-0.06BNZ (Fig. 2a, Figs. S15 and S16). Despite this shared M-T phase coexistence, notable differences emerge between the two systems (Tables S1–S3). KNN-0.06Li exhibits an approximately equal M-to-T phase ratio, with the T phase characterized by a large tetragonality (i.e., c/a=1.016). Conversely, KNN-0.06BNZ exhibits a dominant T phase with smaller tetragonality (i.e., c/a=1.009). Additionally, the lattice parameter (a, b, and c) differences in the M phase are more pronounced in KNN-0.06Li but significantly reduced in the KNN-0.06BNZ. Lower degrees of non-cubic lattice distortion, typically facilitating polarization rotation and domain switching under applied electric fields, imply that KNN-0.06BNZ requires lower exciting voltage for domain switching. This is consistent with the lower coercive field (Ec) and local coercive voltage (Vc) for KNN-0.06BNZ, as observed in P–E loops and litho-PFM measurements, respectively (Fig. 1i and Figs. S9–S11).

The ADPs further suggest increased local atomic disorder in transitioned T phase, reflected by higher equivalent isotropic displacement parameter (Uiso) values for oxygen (Tables S1–S3). This local disorder likely contributes to the M-T phase transition, which is further explored through neutron pair distribution function (PDF). PDF results reveal splitting B–O atomic correlations at 1.9 and 2.1 Å and triplet subpeaks for A–O atomic correlation, indicative of B-site off-center displacement and diverse A–O bonds (Fig. 2b). Despite the long-range M-to-T phase transition in KNN-0.06Li and KNN-0.06BNZ, Rietveld-like refinements on the PDF indicate that the short-range structural symmetry of the M persists in both ceramics (Fig. 2c and Fig. S17). Note that the local structure model with separate treatment of A/B-site atoms and anisotropic displacement parameters could better reproduce the experimental PDF data (Fig. S18). Big-box modeling using reverse Monte Carlo (RMC) further reveals the off-center displacement of specific elements, particularly for Bi and Li atoms (Fig. 2d–f and Figs. S19–S21). Intriguingly, these dopants exhibit fundamentally different displacement behaviors.

The off-center displacements of Bi atoms exhibit multi-directional characteristics, spanning cross [110], [100], and [110] directions, aligning with the spontaneous polarization directions of the M and T phases (Fig. 2e, g). In the KNN system, the spontaneous polarization directions of M and O phases are nearly identical due to the negligible difference between the monoclinic Pm phase and the primary perovskite unit cell orthorhombic Amm2 (see Table S4). Given the high degree of divergence in these off-center displacements, we term this behavior as divergent displacement. In contrast, the off-center displacements of Li atoms are predominantly along [100] and [100] directions (i.e., the spontaneous polarization directions of T phase) and are much more focused than those of Bi atoms (Fig. 2f, h). Thus, we characterize the Li atomic displacements as convergent displacement. Such divergent and convergent displacements of dopants have not been experimentally reported earlier for KNN-based solid-solutions. The RMC analysis also reveal distinct off-center displacements of Nb atoms in KNN-0.06Li and KNN-0.06BNZ, suggesting retained spontaneous polarization from B-site ions (Figs. S20 and S21), consistent with their measured P–E loops (Fig. 1i, j). Besides, Na atoms also show pronounced off-center displacement, whereas K atoms exhibit weaker displacement likely due to the smaller ion radius of Na+ than K+, analogous to previous results21–23. Note that the displacement of Zr atoms is similar to that of Nb atoms, which contrasts with the distinct distributions of Bi, and K/Na (Fig. S21), indicating that the phase transition in KNN-yBNZ is mainly attributed to Bi atoms.

We next evaluate the statistical distribution of A-site and B-site atoms for better explaining the underlying physical mechanisms governing phase transitions in these three KNN solid-solution compositions24. Atomic correlation analysis reveals directional displacement of A-site and B-site atoms relative to their surrounding oxygen atoms, visualized as stereographic projections viewed along the [001] axis (Fig. 3 and Fig. S22). In KNN, B-site atomic displacements predominantly formed a triangular region within [111], [111], and [001] directions, statistically aligning with the [101] direction, characteristic of the M phase symmetry (Fig. 3a). KNN-0.06Li exhibits a distinct pattern, with B-site atoms shifting primarily between the [001] and [101] direction, inducing a displacive phase transition from M to the T phase (Fig. 3b). Conversely, KNN-0.06BNZ exhibits significant B-site disorder, with displacements distributed along multiple directions (i.e., [111], [111], [111], and [111] directions), which averages out to yield a net T phase (i.e., a statistically [001] direction). This pattern reflects an order-disorder transition, culminating in a T phase (Fig. 3c). A-site atomic displacements also exhibit composition-dependent variations. In KNN, A-site atoms displaced predominately along the [110] direction (Fig. 3d); while in KNN-0.06Li, these displacements become more disordered, spanning an orientation space between [110] and [110] directions, facilitating an M-to-T transition with a net [100] displacement direction (Fig. 3e). KNN-0.06BNZ demonstrates even greater A-site atoms disorder, with displacement across four directions (i.e., [110], [110], [110] and [110]), averaging out to a net zero displacement and generating a T phase featured with a low tetragonality (i.e., c/a~1.009) (Fig. 3f).

These results reveal fundamentally different mechanisms driving the M-to-T phase transition in two doped systems. KNN-0.06Li experiences a phase transition driven by a combination of B-site displacive and A-site order-disorder transitions. By contrast, KNN-0.06BNZ undergoes a predominant order-disorder phase transition characterized by increased atomic displacements disorder in both AO and BO polyhedron. The interplay between displacive and order-disorder transitions, governed by the cooperative behavior of A-site and B-site atoms within their polyhedral, determines the average phase structures. These atomic structural changes directly influence mesoscopic ferroelectric domains and ultimately affect the macroscopic electrical properties.

We then analyze the local structure using the TEM/STEM techniques. Analogous to the foregoing PFM measurements, TEM images reveal different ferroelectric domains in two doped systems. Specifically, KNN-0.06Li exhibits abundant stripe and herringbone domains (with a size ranging from tens to hundreds of nanometers) (Fig. 4a and Fig. S23). In contrast, KNN-0.06BNZ shows nano-sized domains (whose domain walls are not situated in specific crystal planes) or even undetectable domains, indicative of disrupted long-range ferroelectric order (Fig. 4d and Fig. S23). To further explore the atomic-scale structure, we measure high-angle annular dark field (HAADF) images and calculate the polarization magnitudes and angles (Fig. 4 and Fig. S24). The projected polarization angle mappings along the [100] direction revealed that both KNN-0.06Li and KNN-0.06BNZ possess the coexisting M(/R) and T phases (Fig. 4b, e, g, i), coinciding with the neutron diffraction results. Quantitative analysis revealed average polarization magnitudes of 19.0 pm for KNN-0.06Li and 13.6 pm for KNN-0.06BNZ (Fig. 4c, f, h, j). The higher polarization amplitude in KNN-0.06Li manifests the larger off-center displacement of both A- and B-site atoms. These STEM-HAADF findings are consistent with our PDF analysis results, providing complementary evidence at the atomic scale and reinforcing our understanding of the structural mechanisms underlying their ferroelectric behaviors.

We then rationalize foregoing experimental observations by using the first-principles calculations based on density functional theory (DFT) and phase-field simulations. Due to the similar off-center displacements of Nb and Zr atoms, only the effect of B ion the crystal structure of KNN is considered. Regardless of whether the matrix symmetry adopts monoclinic Pm or tetragonal P4mm phase configuration, the relaxed supercells consistently reveal a more significant off-center displacement of Li atoms compared to Bi atoms (Fig. 5a–c, Figs. S25 and S26). Compared with pristine KNN supercells, Li atoms significantly increase the disparity in Nb–O bond lengths within the nearest neighboring NbO6 octahedra, while Bi atoms markedly reduce these disparities (Fig. 5d and Fig. S26d). Thus, Li atoms disrupt the short-range order of Nb atoms but retain a displacement of the long-range order of Nb atoms along other directions (e.g., [001] direction); conversely, Bi atoms disrupt both the short-range and long-range order of Nb atoms, consistent with the atomic displacement distributions observed in our PDF analysis. The intensified Nb–O bond length difference in KNN-Li indicates the higher bond strength of Nb–O bonds, whereas the reduced difference in KNN-Bi corresponds to the lower bond strength, well coinciding with the analysis of Raman spectra (Fig. S27). Furthermore, compared with KNN supercells, the calculated density of states (DOS) profile and charge density reveal the negligible hybridization between Li and O atoms in KNN-Li supercells, while the weakened hybridization between Nb and O atoms and the distinct hybridization between Bi and O atoms are observed in KNN-Bi supercells (Fig. 5e–g and Fig. S28). These findings indicate that the off-center displacement of Li atoms arises entirely from its smaller ionic radius relative to K/Na atoms, whereas the displacement of Bi atoms results from a synergistic effect of its small ionic radius and Bi–O hybridization.

To further elucidate the role of mesoscopic domains in enhancing piezoelectricity, we conduct phase-field simulations. In the initial state (i.e., E=0), all three samples show domain configurations analogous to the results of PFM and TEM measurements (Fig. 5h). Under the maximum electric field (i.e., E=Emax), both KNN and KNN-Li show partial domain switching, while KNN-BNZ demonstrates a predominant domain switching, consistent with the litho-PFM results (Fig. S9). The corresponding Landau energy profiles show the most flattened landscape for KNN-BNZ, whereas the deepest one observed for KNN-Li (Fig. 5i). The flattened landscape of KNN-BNZ indicates easier polarization rotation and higher dielectric response, accounting for its higher d33 values. Moreover, the phase-field simulations well reproduce the experimental ferroelectric and strain properties (Fig. S29), proving the reliability of this theoretical calculation.

Methods
The (1−x)(K0.48Na0.52)NbO3−xLiNbO3 and (1−y)(K0.48Na0.52)NbO3−y(Bi0.5Na0.5)ZrO3 ceramics (abbreviated as KNN-xLi and KNN-yBNZ, respectively; x=0, 0.06, 0.07, and 0.075; y=0, 0.045, 0.06, and 0.08) were prepared by the conventional solid-state sintering method. Here, (1−x)(K0.48Na0.52)NbO3−xLiNbO3 can be written as (K0.48Na0.52)1−xLixNbO3. That is to say, Li atoms designedly doped into the A-site, analogous to the situation of (K0.95Li0.05)NbO3 and (Ba1−xLix)TiO3−x/2 ceramics35,36. The subsequent ICP-OES, TOF-SIMS, and XPS results also demonstrate that Li atoms occupy the A-site instead of the B-site (Figs. S1–S3). For (1−y)(K0.48Na0.52)NbO3−y(Bi0.5Na0.5)ZrO3 ceramics, the chemical formula can be written as (A1−0.5yBi0.5y)(Nb1−yZry)O3 (A=K/Na). Therefore, Bi and Zr atoms replace the A-site and B-site atoms and generate the A-site vacancies and oxygen vacancies simultaneously in the following equation26:

0.5Bi2O3 + 2ZrO2 K!NN BiK=Na + 2ZrNb + 2V'K=Na + V••O + 5.5OO

Obviously, the concomitant A-site vacancies and oxygen vacancies occur to remain the charge neutrality overall in KNN-yBNZ ceramics. K2CO3 (99%), Na2CO3 (99.8%), Nb2O5 (99.95%), Li2CO3 (99%), Bi2O3 (99.999%), ZrO2 (99%), and Fe2O3 (99%) (Sinopharm Chemical Reagent Co., Ltd, Shanghai, China) were selected as raw materials. 0.2 mol% Fe2O3 was introduced to promote the sintering of KNN-yBNZ ceramics. A small amount of Fe2O3 mainly acted as a sintering aid and minimal impact on the phase transition temperature37, as evidenced by the fact that our KNN-yBNZ ceramics showed very similar change tendency in phase transition temperature evolution compared with other KNN-BNZ ceramics reported in literature (Fig. S32)38. Moreover, our KNN-yBNZ ceramics showed comparable d33 values but lower T, relative to other KNN-BNZ ceramics38,39. These results confirmed that the Fe2O3 addition did not alter the fundamental physical mechanism discussed in this work. While no sintering aid was needed for KNN-xLi ceramics (x=0.06, 0.07, and 0.075) as Li can reduce the sintering temperature and promote sintering process. After being weighed based on the standard stoichiometric ratio, the powders were ball-milled in the plastic jar for 24 h by the aids of the anhydrous ethanol and zirconia ball media. The dried slurry was calcined at 850 °C for 6 h, and then it was ball-milled again for 24 h to make the mixture more evenly. A moderate amount of 5 wt% polyvinyl alcohol (PVA) was dripped into the fine powders time by time as a binder to form granular clusters, and then they were pressed into disks (1 mm in thickness and 10 mm in diameter) under the applied pressure of near 400 MPa. PVA was burnt off at 850 °C for 3 h, and then green disks were sintered at 1040–1100 °C for 3 h under a corundum crucible in air.

The density of as-sintered ceramics was measured by the Archimedes method, which was widely used in the research of piezoceramics40,41. In this work, we used an electronic balance (with precision of 0.0001 g, BSA224S, Sartorius, Germany) equipped with a set of accessories for measuring density (see Fig. S33). The detailed process is listed as follows. We first cleaned up as-sintered samples and then put them into the drying oven with temperature of 120 °C for 2 h, with the purpose of removing the moisture. Then, we constructed the apparatus shown in Fig. S33. The hollow metal basket was completely immersed into the deionized water. Subsequently, the dry weight (m1) of samples was measured by putting the sample into the top small box. After obtaining the dry weight (m1), the sample was quickly transferred into the hollow metal basket and kept for 30 min, consequently obtaining the wet weight (m2). According to the buoyancy principle (i.e., V=m1−m2, V is the volume of samples and ρ is the density of deionized water (i.e., 1 g·cm−3). Therefore, the density of samples was calculated by the ratio between the measured density and the theoretical density of 4.51 g·cm−342. All sintered ceramic disks show a relative density higher than 95% (see Table S5). Note that the presence of closed pores may affect the accuracy of the Archimedes method, which was the possible reason why some references reported high densities but also observed visible pores40.

X-ray diffraction (XRD) instrument (Bruker D8 Advance XRD, Bruker-AXS Inc., Madison, WI, Cu-Kα) was used to measure the crystal structure of as-sintered samples. Then, samples were ground into fine powders for the neutron diffraction experiments. Temperature-dependent Raman spectra were recorded on a Jobin-Yvon LabRAM HR Evolution micro-Raman spectrometer with excitation sources of 514 nm from an Ar+ laser, in connection with an external temperature control. Polished samples were mechanically with the nano-diamond polishing slurry (DS001, Truer, Shanghai, China). Ferroelectric domains were observed in the contact mode by using a commercial atomic force microscope (MFP-3D, Asylum Research, Goleta, USA) aided by a conductive Pt-Ir-coated cantilever (PPP-NCHPt, Nanosensors, Switzerland). Switching spectroscopy piezoelectric force microscopy (SS-PFM) was obtained by applying an alternating voltage of ±30 V. To detect the domain reversal with the litho-PFM mode, a voltage of −40 V is applied first, and then the gradient-increasing voltage from +5 to +20 V is applied to make some domains switch back to the up. Then, polished samples were thermally etched at a temperature 50 °C lower than the sintering temperature for 30 min. The surface morphology of polished and thermally-etched samples was recorded by a field-emission scanning electron microscopy (FE-SEM, JSM-7500, Japan). The chemical composition of KNN-0.06BNZ ceramics was analyzed by the SEM energy disperse spectroscopy (SEM-EDS). To analyze the chemical composition of KNN-0.06Li ceramics, the inductively coupled plasma optical emission spectrometer (ICP-OES, 5100 SVDV, Agilent, USA) and the time-of-flight secondary ion mass spectrometry (TOF-SIMS, PHI nanoTOF II, ULVAC-PHI, Inc., Japan) measurements were conducted. The X-ray photoelectron spectroscopy measurements (AXIS Supra, Kratos, UK) were measured to evaluate the oxygen vacancy.

Temperature- and frequency-dependent dielectric properties (i.e., the real part of dielectric constant, the imaginary part of dielectric constant and the dielectric loss) of unpoled (i.e., ε′, ε″, and tan δ) and poled samples (i.e., ε330, ε3300, and tan δ) were measured by a broadband dielectric spectrometer (Novocontrol Concept 80, Novocontrol Technologies GmbH, Germany) over the temperature range of −150–250 °C and −260–0 °C. The temperature-dependent relative dielectric permittivity (εr) was measured at 30–550 °C by using the dielectric spectroscopy test system (TZDM-RT-800, Harbin Julang Technology Co Ltd., Harbin, China). Polarization-electric field (P–E) loops and strain-electric field (S–E) curves were collected by using a commercial ferroelectric workstation (aixACCT TF Analyzer 2000, Germany) with a laser interferometer vibrometer (SP-S120/500, SIOS Meßtechnik GmbH, Germany). The small signal piezoelectric coefficient (d33) was obtained from a quasi-static d33 meter (ZJ-3A, China Academy of Acoustics, China).

The temperature-dependent synchrotron X-ray diffraction measurements of KNN-0.06Li and KNN-0.06BNZ ceramic powder samples were conducted on BL02U2 instrument of Shanghai Synchrotron Radiation Facility (SSRF). The wavenumber of beamline is about 1.27 Å, and the beam energy is about 10 keV.

The sintered ceramic samples were cut, grinded, and polished to pellets of 3 mm in diameter and 30 μm in thickness followed by ion-milling of Ar+ using a Gatan PIPS II (Gatan Inc., Pleasanton, CA) from 3 keV to 0.5 keV for reducing ion-beam damage. The perforated thin disc was coated with a thin carbon film to reduce charge accumulation during electron beam illumination in TEM experiments. A JEM-F200 microscope was used to acquire dark-field images and selected area electron diffraction (SAED) patterns. The atomic-scale high-angle annular dark-field (HAADF) imaging was carried out on a Cs-corrected Hitachi HF5000 microscope with ultra-high resolution (UHR) mode and a convergence/collection semi-angle of 20 mrad/60‒320 mrad. The atomic column positions at picometer-precision fitting were operated by MATLAB code43,44.

Density functional theory (DFT) calculations were implemented in the framework of Vienna ab initio Simulation Package (VASP) in conjunction with the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation based on the generalized gradient approximation (GGA)45–49. The projector augmented wave (PAW) was utilized to describe the electron-ion interactions and plane wave cutoffs were set to 500 eV. The valence electron configurations were K 3s3p4s, Na 2p3s, Nb 4s4p4d5s, Li 1s, Bi 5d6s6p, O 2s2p states. Two KNN structures, one with the Pm phase and the other with the P4mm phase, were considered, respectively. In the case of Pm phase, the supercell size is 2×√2×3√2 and contains 60 atoms. In the case of P4mm phase, the supercell size is 2×2×4 and contains 80 atoms. For Li- and Bi-doped supercells, each supercell contains one Li or Bi atom that replaced one Na atom. The relaxed supercell configurations were depicted in Fig. S25. All models were exhibited using the 6×9×3 Monkhorst-Pack k-point meshes50. The convergence criteria for electronic self-consistent iterations were set to 10−6 eV, and atomic coordinates were optimized until the Hellmann-Feynman forces were less than 0.001 eV/Å. The simulation data was analyzed by employing VASPKIT, a pre- and post-processing tool for the VASP code.

Time-of-flight (TOF) neutron scattering experiments were performed at the Nanoscale-Ordered Materials Diffractometer (NOMAD) BL1B instrument of the Spallation Neutron Source (SNS)51. The synthesized powder samples were loaded into vanadium cans of a diameter of ~6 mm, which were placed on an automatic sample changer for sequential data collection. For each sample, two 24 min long datasets (comparable proton charges) were collected and then summed together to improve the measurement statistics. Background intensity measured from an empty vanadium can was subtracted from the neutron scattering intensity measured from each sample. The background-corrected intensity was subsequently normalized against the signal from a 6 mm vanadium rod to correct for detector efficiency. Self-scattering was subtracted during the data reduction, and the data was not absolutely normalized. The intensities were normalized and reduced to the structure function S(Q). To obtain the corresponding reduced PDFs, the S(Q) data were then Fourier-transformed using the function:

G(r) = 2/π ∫ QminQmax Q[S(Q)−1] sin(Qr) dQ (1)

where Qmin = 0.2 Å−1 and Qmax = 40 Å−1.

Rietveld refinements of Bragg diffraction from the middle four banks, corresponding to 2θ of 31°, 67°, 122°, and 154°, were carried using the General Structure Analysis System (GSAS-II) software20. PDF small-box modeling was conducted using the software PDFgui52. Big-box modeling was conducted using RMCprofile53. Starting configurations for all compositions were generated based on the best fit of monoclinic (M) model to Bragg diffraction data. The modeling boxes contains 12×12×12 unit cells, 8640 atomic sites, with dimensions of approximately 48×48×48 Å. Fits to the PDF and F(Q) data were conducted, fixing PDF data fits across 1.5–10 Å. A “distance-window” constraint was applied to avoid unphysical pair distances, and bond valence sum restraints were used to minimize unphysical bonding configurations. Swapping was allowed between different cations occupying the same sites in the ABO3 structure. To ensure statistical reliability, RMC refinements were performed 20 times to assess the consistency of the final results.

To investigate the intricate domain configurations present in randomly arranged grain configurations, an order parameter η was introduced to encapsulate the wide range of crystal orientations. To capture the polarization characteristics within these diverse grain structures, two distinct coordinate systems were adopted. Within each individual grain, the domain structure manifests as a locally spontaneous polarization phenomenon. Transitioning to a broader perspective, the phase-field model incorporated key order parameters such as global polarization P and displacement field u. The temporal evolution of polarization was tracked by utilizing the time-dependent Ginzburg-Landau (TDGL) equation, in conjunction with the stress/electric field equilibrium equation,

∂Pi/∂t = −L δF/δPi + Ethermal (2)

∇·(σij) = 0

Here, L represents a kinetic coefficient associated with domain wall mobility, while F denotes the overall free energy of the system. The thermodynamic driving force is represented by δF, and σ signifies the stress tensor. Additionally, D stands for the electric displacement, and ρf represents the free charge density. r and t are the spatial coordinate and time, respectively. The total free energy of a bulk system can be formulated as follows,

F = Fland(P) + Fgrad(P) + Felastic(P) + Felec(P,E) = ∫V (fland + fgrad + felastic + felec) dV (3)

Where F includes the bulk free energy Fbulk(P), domain-wall energy Fgrad(P), elastic energy Felastic(P), and electrostatic energy Felec(P,E). fland, fgrad, felastic and felec are the corresponding energy densities. The bulk free energy density within a specific grain can be expressed as an expansion in terms of polarization components. In the context of the KNN system, the bulk free energy density can be comprehensively described as follows:

fland = a1(P12 + P22 + P32) + a11(P14 + P24 + P34) + a12(P12P22 + P12P32 + P22P32) + a111(P16 + P26 + P36) + a1112[P6(P2 + P2) + P6(P2 + P2) + P6(P2 + P2)] + a1122(P4P4 + P4P4 + P4P4) + a1123(P4P12P22 + P12P4P22 + P12P22P4) (4)

fLand incorporates the Landau coefficients a1–a1123 and the polarization field P in the local crystallographic coordinate system within each grain. The polarization within the local coordinate system can be formulated as a transformation arising from the polarization expressed in the global coordinate system, achieved through the application of a transformation matrix,

PLi = trij Pj (5)

The transformation matrix tr facilitates the transition from the global to the local coordinate system,

tr = [cosφcosψ−cosθsinφsinψ, sinφcosψ+cosθcosφsinψ, sinθsinψ; −cosθcosψsinφ−cosφsinψ, cosθcosφcosψ−sinφsinψ, sinθcosψ; sinθsinφ, −cosφsinθ, cosθ] (6)

The gradient energy density in an anisotropic system can be calculated by

fgrad = 1/2 gijkl Pi,j Pk,l (7)

where gijkl is the gradient energy coefficient and Pi,j = ∂Pi/∂xj. The elastic energy density can be described as:

felas = 1/2 cijkl eij ekl = 1/2 cijkl (εij−ε0ij)(εkl−ε0kl) (8)

where cijkl is the stiffness tensor, eij is the elastic strain tensor, εij is the total strain tensor, and ε0ij is the eigenstrain,

ε0ij = Qijkl Pk Pl (9)

The eigenstrain in the global coordinate system can be derived by employing an appropriate transformation matrix,

ε0ij = trki trlj ε0kl (10)

The electrostatic energy density felec of the system in phase-field simulation is given by,

felec = −P(r)·(E(r) + E) − 1/2 P(r)Ein(r) (11)

where Ein(r) is the E-field induced by the dipole moments, E(r) is the applied electric field and E is the local electric field caused by the random point defects. The parameters (all in SI units) in details are a1 = 4.12 × 109, a11 = −3.04 × 109, a12 = 1.72 × 109, a111 = 2.73 × 109, a1111 = 2.4 × 1010, a1112 = 3.73 × 109, a1122 = 3.34 × 1010, a1123 = −6.2 × 1010, Q11 = 0.13, Q12 = −0.047, Q44 = 0.052, s11 = 5.5 × 10−11, s12 = −1.6 × 10−10, s44 = 1.3 × 10−9. Here, s represents the compliance coefficients, and Q denotes the electrostrictive coefficients. The simulation scale encompassed a domain of 512dx×512dz, with grid size 10 nm. For the grain structure, a random orientation was assigned to each grain. To solve the equations, the Fourier method was utilized. Furthermore, the calculations incorporated the open-circuit electrical boundary condition, in conjunction with periodic mechanical boundary conditions, ensuring accurate and comprehensive simulations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Comments on the manuscript “Beyond phase boundaries: atomic mechanisms governing structure and property variations in (K, Na)NbO3-based ferroelectrics by Xin Wang, Jing Kong, Xiaoming Shi, Hou-bing Huang, Tengfei Hu, Zhengqian Fu, Jing Lyu, Yinchang Ma, Xi-xiang Zhang, Bo Wu, Abhijit Pramanick, Xiang Lv, and Jiagang Wu.\nIn the present manuscript, the authors presented a comprehensive multi-scale structural studies to explain the role of chemical dopants towards construction of the phase boundaries in KNN-based ceramics. Sodium potassium niobate (KNN) modified by Ta, Li and Sb attracted some interest since they presented room-temperature piezoelectric constants comparable to that of PZT. The studies were done for two representative solid-solution systems: (1-x)(K0.48Na0.52)NbO3-xLiNbO3 and (1-y)(K0.48Na0.52)NbO3-y(Bi0.5Na0.5)ZrO3 (KNN-xLi and KNN-yBNZ) by utilizing several-technique investigation: neutron total scattering, scanning transmission electron microscopy (STEM), first-principles calculations, and phase-field simulations. Based on obtained results, the authors shown that atomic-scale structural characteristics directly correlate with mesoscopic ferroelectric domains and determine macroscopic electrical properties. Thus, they find the “trait d’union” between short-range structure and long-range behavior of investigated materials. I thing that it is a significant advancement beyond the state-of-the-art, not only for investigated materials but also for lead-free ferroelectrics.\nThis article is interesting for readers and fits the scope of Nature Communications. However, some improvements are needed:\n- Abstract part, is “Here, elucidate these mechanisms through a comprehensive multi-scale structural analysis (atomic-to-nanoscale-to-mesoscale) on two representative solid-solutions, namely (K, Na, Li)NbO3 and (K, Na)NbO3-(Bi0.5Na0.5)ZrO3”, should be “Here, explanation these mechanisms through a comprehensive multi-scale structural analysis (atomic-to-nanoscale-to-mesoscale) on two representative solid-solutions, namely (K, Na, Li)NbO3 and (K, Na)NbO3-(Bi0.5Na0.5)ZrO3“ was given.\n- Introduction part, is “In this work, we conduct a comprehensive multi-scale structural analysis, with particular emphasis on local atomic scale, to decipher the role of chemical dopants towards construction of the phase boundaries in KNN-based ceramics”, should be “In this work, we conduct a comprehensive multi-scale structural analysis, with particular emphasis on local atomic scale, to explain the role of chemical dopants towards construction of the phase boundaries in KNN-based ceramics”,\n- Results (neutron total scattering analysis )part, is “We next evaluate the statistical distribution of A-site and B-site atoms for better deciphering the physical mechanisms underlying the phase transition in these three KNN solid-solution compositions24.”, should be “We next evaluate the statistical distribution of A-site and B-site atoms for better explaining the physical mechanisms underlying the phase transition in these three KNN solid-solution compositions24”,\nResults (neutron total scattering analysis) part, is “Noting that the displacement of Zr atoms is similar to that of Nb atoms, which contrasts with the distinct distributions of Bi, and K/Na (Fig. S17), indicating that the phase transition in KNN-yBNZ is mainly attributed to Bi atoms”, should be “Note that the displacement of Zr atoms is similar to that of Nb atoms, which contrasts with the distinct distributions of Bi, and K/Na (Fig. S17), indicating that the phase transition in KNN-yBNZ is mainly attributed to Bi atoms”.", "reviewer_2": "Review of the Manuscript by Wang et al.\nThe manuscript by Wang et al. presents a comprehensive structural analysis of lithium (Li)-doped (K,Na)NbO3 ceramics and (K,Na)NbO3(Bi0.5Na0.5)ZrO3 solid solutions. The study integrates average structural characterization via X-ray and neutron diffraction, pair distribution functon(PDF) analysis, local structural analysis using transmission electron microscopy (TEM), as well as theoretical investigations using density functional theory (DFT) and phase-field modeling.\nWhile the impact of chemical doping on phase stability in these material systems is certainly of interest to the materials science community, the findings presented here do not appear to offer a significant advance or novel insight in either fundamental science or engineering applications that would merit publication in Nature Communications. Moreover, the manuscript lacks clarity in its technical descriptions and requires substantial improvements to enhance its accessibility and scientific rigor. Some conclusions drawn by the authors also seem insufficiently supported by the data presented.\nI therefore cannot recommend this manuscript for publication in its current form. The manuscript has potential but falls short in terms of novelty, clarity, and the strength of its conclusions. If the authors can clarify and revise the manuscript substantially - especially addressing the structural modeling, data interpretation, and the physical basis of the claims - then the work may be more suitable for a different journal with a more specialized audience.\nSpecific Comments\nOn Grain Size and Relative Density\nThe SEM images in Fig. S9 for both KNN and KNN-0.06Li show a wide distribution of grain sizes, including abnormally large grains alongside finer ones. In this context, reporting a simple average grain size is not meaningful. The authors should reconsider their interpretation and possibly provide a grain size distribution or median values.\nFurthermore, the SEM images for KNN-0.06BNZ and KNN-0.045BNZ show a considerable number of pores, which appears inconsistent with the reported high relative densities (> 95%) in Table S4. This raises concerns about the reliability of the relative density measurements. The authors should provide more details on the measurement method used and discuss this inconsistency.\nNeutron Diffraction and PDF Analysis\nThe structural models could potentially be improved by considering the individual positions of K, Na, Li, and Bi ions, rather than averaging over site occupancies. Similarly, separate treatment of Zr and Nb positions may enhance the model's fidelity. Additionally, anisotropic displacement parameters (ADPs) may be more appropriate than isotropic ones in capturing the complexity of local structure. A comparison of such refined models with the PDF data would strengthen the analysis.\nThe manuscript uses terms like “divergent displacement” for Bi ions and “convergent displacement” for Li ions, which are unclear. Please revise these expressions to clarify the physical meaning or replace them with more standard terminology.\nOn page 9, the statement “note that the spontaneous polarization directions of M and O phases are the same in the KNN system” appears questionable. In perovskite-type compounds, the spontaneous polarization in the orthorhombic phase (e.g., BaTiO3) is typically along the <110> direction, while in the monoclinic Pm phase (e.g., PMN-0.32PT), it is commonly along a general [h0l] direction. The authors should either revise this statement or provide appropriate citations.\nDensity Functional Theory (DFT) Calculations\nIn Fig. 5a-c, the displacement of cations is not clearly visible. It would be helpful if the authors could enhance the visibility of atomic displacements (e.g., using arrows or color coding). Additionally, the manuscript should specify the number of Li or Bi ions included in the supercell and how their configurations were modeled.", "reviewer_3": "The manuscript presents a comparative investigation of (K,Na)NbO₃-based ferroelectrics modified with Li and (Bi₀.₅Na₀.₅)ZrO₃, with the stated goal of uncovering atomic mechanisms responsible for phase boundary formation and their influence on macroscopic properties. While the study combines multiple experimental and theoretical approaches, several critical scientific and methodological flaws fundamentally undermine the validity of the authors' conclusions. These issues, detailed below, reflect insufficient rigor in both the experimental design and interpretation:\n1. The chemical composition of the synthesized samples should be clearly confirmed. This is particularly important for the Li content, as Li is a light element and its precise quantification is typically challenging. The authors are encouraged to provide supporting evidence (e.g., ICP, TOF-SIMS, or other sensitive techniques).\n2. In the KNN structure, Li⁺ ions can occupy either the A-site or B-site. The manuscript does not provide sufficient evidence to distinguish between these possibilities. How did the authors determine the preferred site(s) of substitution and quantify the distribution of Li between the A- and B-sites?\n3. The substitution of Bi³⁺ into the KNN lattice introduces valence imbalance. The authors should elaborate on the mechanism of charge compensation. Specifically, what is the role of A-site vacancies in maintaining charge neutrality?\n4. In the case of Li substitution, particularly if Li occupies the B-site, oxygen vacancies may form to maintain charge balance. This aspect should be discussed in greater depth, ideally with supporting evidence from techniques such as XPS or other methods capable of evaluating defect chemistry or charge compensation.\n5. The experimental results suggest the coexistence of monoclinic (Pm) and tetragonal (P4mm) phases. Therefore, DFT calculations should explicitly consider both symmetries to ensure the accuracy and consistency of the theoretical predictions.\n6. In the experimental section, 0.2 mol% Fe₂O₃ was added to promote sintering in KNN-yBNT ceramics, whereas KNN-Li samples were synthesized without Fe₂O₃. Given that Fe³⁺ ions can influence electrical and structural properties even in small amounts, how did the authors confirm that Fe³⁺ was not unintentionally introduced into the KNN-Li samples, or that its presence did not affect the observed properties?"}, "author_response": "Response to Reviewers\n(Manuscript no: NCOMMS-25-48604)\nResponse to Reviewer #1:\nComments on the manuscript “Beyond phase boundaries: atomic mechanisms governing structure and\nproperty variations in (K, Na)NbO -based ferroelectrics by Xin Wang, Jing Kong, Xiaoming Shi, Hou-\n3\nbing Huang, Tengfei Hu, Zhengqian Fu, Jing Lyu, Yinchang Ma, Xi-xiang Zhang, Bo Wu, Abhijit\nPramanick, Xiang Lv, and Jiagang Wu.\nIn the present manuscript, the authors presented a comprehensive multi-scale structural studies to\nexplain the role of chemical dopants towards construction of the phase boundaries in KNN-based\nceramics. Sodium potassium niobate (KNN) modified by Ta, Li and Sb attracted some interest since\nthey presented room-temperature piezoelectric constants comparable to that of PZT. The studies were\ndone for two representative solid-solution systems: (1-x)(K Na )NbO -xLiNbO and (1-\n0.48 0.52 3 3\ny)(K Na )NbO -y(Bi Na )ZrO (KNN-xLi and KNN-yBNZ) by utilizing several-technique\n0.48 0.52 3 0.5 0.5 3\ninvestigation: neutron total scattering, scanning transmission electron microscopy (STEM), first-\nprinciples calculations, and phase-field simulations. Based on obtained results, the authors shown that\natomic-scale structural characteristics directly correlate with mesoscopic ferroelectric domains and\ndetermine macroscopic electrical properties. Thus, they find the “trait d’union” between short-range\nstructure and long-range behavior of investigated materials. I thing that it is a significant advancement\nbeyond the state-of-the-art, not only for investigated materials but also for lead-free ferroelectrics. This\narticle is interesting for readers and fits the scope of Nature Communications. However, some\nimprovements are needed:\nReply: Thanks for your kind recognition and recommendation of our work. We have revised the\nmanuscript according to your suggestions.\nComment 1. Abstract part, is “Here, elucidate these mechanisms through a comprehensive multi-scale\nstructural analysis (atomic-to-nanoscale-to-mesoscale) on two representative solid-solutions, namely\n(K, Na, Li)NbO and (K, Na)NbO -(Bi Na )ZrO ”, should be “Here, explanation these mechanisms\n3 3 0.5 0.5 3\nthrough a comprehensive multi-scale structural analysis (atomic-to-nanoscale-to-mesoscale) on two\nR-1\nrepresentative solid-solutions, namely (K, Na, Li)NbO and (K, Na)NbO -(Bi Na )ZrO “ was given.\n3 3 0.5 0.5 3\nReply: Thanks for your comment and sorry for the grammar mistake. We have revised the sentence,\nas shown below.\n(Page 2 in the revised Manuscript)\nComment 2. Introduction part, is “In this work, we conduct a comprehensive multi-scale structural\nanalysis, with particular emphasis on local atomic scale, to decipher the role of chemical dopants\ntowards construction of the phase boundaries in KNN-based ceramics”, should be “In this work, we\nconduct a comprehensive multi-scale structural analysis, with particular emphasis on local atomic scale,\nto explain the role of chemical dopants towards construction of the phase boundaries in KNN-based\nceramics”,\nReply: Thanks for your comment. We have revised “decipher” to “explain” on page 4 as suggested, as\nshown below.\n(Page 4 in the revised Manuscript)\nComment 3. Results (neutron total scattering analysis) part, is “We next evaluate the statistical\ndistribution of A-site and B-site atoms for better deciphering the physical mechanisms underlying the\nphase transition in these three KNN solid-solution compositions24.”, should be “We next evaluate the\nstatistical distribution of A-site and B-site atoms for better explaining the physical mechanisms\nunderlying the phase transition in these three KNN solid-solution compositions24”,\nReply: Thanks for your comment. We have revised the manuscript according to your suggestion, as\nshown below.\nR-2\n(Page 11 in the revised Manuscript)\nComment 4. Results (neutron total scattering analysis) part, is “Noting that the displacement of Zr\natoms is similar to that of Nb atoms, which contrasts with the distinct distributions of Bi, and K/Na\n(Fig. S17), indicating that the phase transition in KNN-yBNZ is mainly attributed to Bi atoms”, should\nbe “Note that the displacement of Zr atoms is similar to that of Nb atoms, which contrasts with the\ndistinct distributions of Bi, and K/Na (Fig. S17), indicating that the phase transition in KNN-yBNZ is mainly\nattributed to Bi atoms”.\nReply: Thanks for your comment. We have revised the manuscript according to your suggestion, as\nshown below.\n(Page 9 in the revised Manuscript)\nR-3\nResponse to Reviewer #2:\nThe manuscript by Wang et al. presents a comprehensive structural analysis of lithium (Li)-doped (K,\nNa)NbO ceramics and (K, Na)NbO -(Bi Na )ZrO solid solutions. The study integrates average\n3 3 0.5 0.5 3\nstructural characterization via X-ray and neutron diffraction, pair distribution function (PDF) analysis,\nlocal structural analysis using transmission electron microscopy (TEM), as well as theoretical\ninvestigations using density functional theory (DFT) and phase-field modeling.\nWhile the impact of chemical doping on phase stability in these material systems is certainly of interest\nto the materials science community, the findings presented here do not appear to offer a significant\nadvance or novel insight in either fundamental science or engineering applications that would merit\npublication in Nature Communications. Moreover, the manuscript lacks clarity in its technical\ndescriptions and requires substantial improvements to enhance its accessibility and scientific rigor.\nSome conclusions drawn by the authors also seem insufficiently supported by the data presented.\nI therefore cannot recommend this manuscript for publication in its current form. The manuscript has\npotential but falls short in terms of novelty, clarity, and the strength of its conclusions. If the authors\ncan clarify and revise the manuscript substantially - especially addressing the structural modeling,\ndata interpretation, and the physical basis of the claims - then the work may be more suitable for a different\njournal with a more specialized audience.\nReply: Thanks for your thorough review and constructive comments. In regard to the “significant\nadvance or novel insight” you commented, we’d like to emphasize them from the following points.\n(1) Significance of lead-free piezoceramics\nPiezoelectric ceramics (i.e., ferroelectric ceramics) are critical for sensors, ultrasonic transducers,\nand actuators, with significance recently highlighted in Science journal [see Science 2025, 389 (6755),\neadn4926]. While lead-based piezoceramics dominate the market, their toxicity urgently necessitates\nlead-free alternatives [see MRS Bull. 2018, 43 (8), 576; Nat. Commun. 2025, 16 (1), 7101]. KNN-based\npiezoceramics represent the most promising candidates due to their excellent piezoelectric\nperformance [see Chem. Soc. Rev. 2020, 49 (3), 671; MRS Bull. 2018, 43 (8), 607].\n(2) Knowledge gap: existing mechanisms cannot explain performance differences\nPrevious studies proposed several mechanisms to explain enhanced piezoelectricity in KNN\nceramics: nano-sized ferroelectric domains (with a scale ranging from 20 nm to 1 μm), room-\ntemperature multi-phase coexistence (with a scale of >10 μm), and electric-induced phase transition\nR-4\n(with a scale of >10 μm) (see Figure R1(b)). However, these meso-/macroscopic-scale mechanisms\nfail to explain a fundamental question: why do KNN-0.06Li and KNN-0.06BNZ, despite having\nsimilar M-T phase boundaries, exhibits markedly different electrical properties? [see J. Mater. Chem. A\n2020, 8 (20), 10026]. Critically, comprehensive reports on how chemical dopants affect the local atomic\nstructure in KNN ceramics remain absent, creating a knowledge gap in the multi-scale structure-\nproperty. This gap keeps current research being in a trial-and-error stage and lacking rational design\nprinciples and predictive capability. Therefore, comprehensive atom-scale understanding is essential\nto advance the development of KNN ceramics.\n(3) Key advances of this work: revelation of dopant-specific atomic displacement mechanisms\nUsing neutron pair distribution function (PDF) analysis — a technique capable of probing local\natomic structure in bulk materials [see Nucl. Instrum. Methods Phys. Res. Sect. B-Beam Interact. Mater.\nAtoms 2012, 287, 68] — combined with PFM, STEM, DFT and phase-field simulations, we reveal the\natomic-scale origin of performance difference in KNN-based piezoceramics: Li-doped KNN and\n(Bi Na )ZrO -doped KNN. Both chemical dopants were widely reported to regulate the\n0.5 0.5 3\northorhombic/monoclinic-tetragonal (O/M-T) phase transition to room temperature and thereby\nachieve the enhanced piezoelectric performance [see J. Mater. Chem. C 2019, 7 (15), 4284; Acta Mater.\n2024, 266, 119691]. We revealed that Li and Bi atoms exhibit fundamentally different off-center\ndisplacement behaviors, which leads to different phase-transition mechanisms, resultant mesoscopic\ndomains and macroscopic performance (see Figure R1(c)). These observations have never been\nreported so far in KNN-based piezoceramics and unambiguously point out the deeper physical\nmechanisms beyond the conventional phase boundary.\n(4) Scientific and practical impact\nOur findings establish atomic-scale design principles for lead-free piezoceramic beyond empirical\napproaches. This represents a fundamental advance in understanding structure-property relationships\nand provides rational guidelines for engineering high-performance lead-free piezoceramics.\n[Figure Redacted]\nFigure R1. (a) Multi-scale structures of a piezoceramic. (b) Currently prevailing physical mechanisms\nexplaining the enhanced piezoelectricity in KNN-based ceramics. (c) Our locally atomic findings.\nR-5\nComment 1. On Grain Size and Relative Density\nThe SEM images in Fig. S9 for both KNN and KNN-0.06Li show a wide distribution of grain sizes,\nincluding abnormally large grains alongside finer ones. In this context, reporting a simple average\ngrain size is not meaningful. The authors should reconsider their interpretation and possibly provide a\ngrain size distribution or median values.\nReply: Thanks for your insightful comment.\nWe completely agree that simple average grain size measurements are inadequate for materials\nexhibiting wide grain size distributions [see J. Appl. Phys. 2021, 129 (2), 024102]. We have provided\ncomprehensive grain size distributions and calculated the median value (i.e., d ), as shown in Figure\n50\nceramics exhibit a significantly larger d than others, which was also observed by several independent\n50\ngroups [see Acta Mater. 2024, 266, 119691; Ceram. Int. 2012, 38 (1), 147; Curr. Appl. Phys. 2011, 11 (3),\nS149].\nFigure R2. (also Fig. S12 in the revised Supplementary Information) FE-SEM images and\nR-6\ncorresponding grain size distributions of polished and thermally-etched (a) KNN, (b) KNN-0.06Li, (c)\nKNN-0.06BNZ, and (d) KNN-0.045BNZ ceramics. Since the much larger grains in KNN-0.06Li, we\nused a FE-SEM image with a low magnification to make more grains being considered. Note that the\ngrain size distribution in (d) corresponds to the left FE-SEM image. The right FE-SEM image in (d)\nshows the rarely abnormally large grain of KNN-0.045BNZ ceramics.\nFurthermore, the grain size analysis strengthens our fundamental argument. Despite KNN-\n0.06Li having the largest d , it exhibits lower dielectric and piezoelectric properties, as we emphasized\n50\nin Figure 1. This contradicts established grain size-permittivity correlations where larger grains\ntypically enhance dielectric properties through improved polarization switching and domain wall\nmotion [see J. Appl. Phys. 2025, 138 (8), 084101; Adv. Electron. Mater. 2019, 5 (12), 1900570; Acta Mater.\n2024, 276, 120112]. This grain size paradox directly supports our central hypothesis that atomic-scale\nmechanisms govern the performance differences between these systems. Finally, we have revised\nFigure S9 and the related analysis, as shown on page S-14 of the revied Supplementary Information.\nComment 2. Furthermore, the SEM images for KNN-0.06BNZ and KNN-0.045BNZ show a\nconsiderable number of pores, which appears inconsistent with the reported high relative densities (>\n95%) in Table S4. This raises concerns about the reliability of the relative density measurements. The\nauthors should provide more details on the measurement method used and discuss this inconsistency.\nReply: Thanks for your insightful comment regarding the inconsistency between our SEM and\nreported relative densities.\nFirst, the SEM images of all ceramics in this work were measured on polished and thermally-\netched samples. This process has been widely used in the research of piezoceramics. The observed\npore mainly stemmed from the shedding of small grains during the polishing and thermal etching\nprocess, a phenomenon commonly encountered in KNN-based piezoceramics, particularly those\nexhibiting a bimodal grain size distribution [see Acta Mater. 2024, 266, 119691; J. Appl. Phys. 2021, 129\n(2), 024102; J. Eur. Ceram. Soc. 2017, 37 (5), 2083]. For instance, considerable pores were still observed\nin the polished and thermally-etched KNN ceramics prepared by the hot-press sintering, despite the\ntested samples exhibiting a relatively high density of up to 98.6-99.8% (see Figure R3).\nR-7\n[Figure Redacted]\nFigure R3. SEM images of (a) KNN ceramics prepared by using hot-pressed sintering (from J. Appl.\nPhys. 2021, 129 (2), 024102) and (b) KNN-xLi ceramics (from Acta Mater. 2024, 266, 119691). All images\nwere taken from the polished and thermally-etched samples. Clearly, these ceramics also show the\nconsiderable number of pores.\nSecond, it was reported that undoped KNN ceramics can have a relative density of 95.1% [see J.\nAm. Ceram. Soc. 2015, 98 (6), 1698]. After doping appropriate Li, Bi, and Fe elements, the relative density\ncan be further improved because the raw materials of these elements can promote the sintering process.\nHere, we also measured SEM images for the surface and cross section of as-sintered samples (i.e.,\nwithout polishing and thermal etching) (see Figure R4). All samples exhibit the relatively dense\nmicrostructure. Analogous to the results of polished and thermally-etched samples, for the surface of\nas-sintered samples, KNN-0.06Li ceramics exhibited the much larger grain size compared with KNN\nand KNN-0.06BNZ ceramics.\nFigure R4. SEM images of the surface and cross section of as-sintered KNN-based ceramics. The red\nscale bar is 5 μm, and all SEM images have the same scanning size. All samples show a transcrystalline\nrupture in their cross-section.\nThird, we measured the density by using the Archimedes method, which was widely used in the\nresearch of piezoceramics [see J. Appl. Phys. 2021, 129 (2), 024102; J. Eur. Ceram. Soc. 2017, 37 (5), 2083].\nIn this work, we used an electronic balance (with precision of 0.0001 g, BSA224S, Sartorius, DE)\nR-8\nequipped with a set of accessories for measuring density (see Figure R5). The detailed process is listed\nas followings. We first cleaned up as-sintered samples and then put them into the drying oven with\ntemperature of 120 ℃ for 2 hours, with the purpose of removing the moisture. Then, we constructed\nthe apparatus shown in Figure R5. The hollow metal basket was completely immersed into the\ndeionized water. Subsequently, the dry weight (m ) of samples was measured by putting the sample in\n1\nthe top small box. After obtaining the dry weight (m ), the sample was quickly transferred into the\n1\nhollow metal basket and kept for 30 minutes, consequently obtaining the wet weight (m ). According\n2\nto the buoyancy principle (i.e., , V is the volume of samples and ρ is the density of\n𝑚1−𝑚2\ndeionized water (i.e., 1 g/cm3). T𝑉he=refo𝜌r𝑤e𝑎,𝑡 𝑒t𝑟he density of samples was calculated by the equation of\n. For each composition, the density was determined by averaging five samples.\n𝑚1 𝑚1\n𝜌T 𝑠 h 𝑎 e 𝑚 r 𝑝 e 𝑙𝑒 la=tive𝑉 d=en s𝑚it1y− 𝑚w2as calculated by the ratio between the measured density and the theoretical\ndensity of 4.51 g/cm3 [see J. Am. Ceram. Soc. 1962, 45 (5), 209].\nFinally, in accordance with your comment, we have added more details on the measurement\nmethod and included the error analysis, as shown on pages 21-22 of the revised Manuscript and on\npages S-35 and S-38 of the revised Supplementary Information.\nSide view of schematic diagram Actual picture of apparatus\nStep 1: Measuring\ndry weight (m) Sample\nmetal\nbasket\nHollowmetal Holder\nStep 2: Measuring basket\nwet weight (m) Sample\nCombination of\nhollow metal basket\nHolder\nand holder\nElectronic balance\nBSA224S (Sartorius, DE)\nFigure R5 (also Fig. S33 in the revised Supplementary Information). The schematic diagram (left)\nand actual picture (right) of the Archimedes method.\nComment 3. Neutron Diffraction and PDF Analysis\nThe structural models could potentially be improved by considering the individual positions of K, Na,\nR-9\nrefsnarT\nLi, and Bi ions, rather than averaging over site occupancies. Similarly, separate treatment of Zr and\nNb positions may enhance the model's fidelity. Additionally, anisotropic displacement parameters\n(ADPs) may be more appropriate than isotropic ones in capturing the complexity of local structure. A\ncomparison of such refined models with the PDF data would strengthen the analysis.\nReply: Thanks for your insightful comment regarding structural model optimization.\nFollowing your suggestions, we conducted systematic comparation between different structural\nmodelling approaches for neutron diffraction and PDF analysis. Initially, we performed Rietveld\nrefinement on the neutron diffraction data, using both averaged site occupancies with isotropic\ndisplacement parameters and individual atomic positions and anisotropic displacement parameters\n(ADPs). However, the anisotropic ADP treatment did not yield significant improvements in fitting\nquality compared to the isotropic approach, as illustrated in Figure R6.\nNevertheless, your suggestion proved highly valuable for PDF analysis. We implemented\nindividual treatment of K, Na, Li, and Bi atoms at A-sites, and separate consideration of Nb and Zr\natoms at B-sites, with anisotropic displacement parameters. This approach yields improved\nreproduction of experimental PDF data (see Figure R7), with reduced residual values. This\ncomparative analysis reveals an important insight: while Bragg diffraction is relatively insensitive to\nthe detailed individual atomic positions due to statistical averaging, PDF analysis benefits significantly\nfrom separate atomic treatment because it captures local pair correlations that are sensitive to specific\natomic environments. We have therefore revised the manuscript to reflect this enhanced structural\nmodel.\nR-10\nFitting using averaged site occupancies and isotropic displacement parameters\n6 8\n6 6 4\n4 4\n2\n2 2\n0\n0\n0\n-2 -2\n-2\n4 5 6 7 8 9 10111213141516 4 5 6 7 8 9 10 11 12 4 5 6 7 8 9 10 11 12\nFitting using individual atomic positions and anisotropic displacement parameters\nFigure R6. Neutron diffraction patterns measured at BANK5 of the NOMAD instrument.\nFigure R7. Experimental and fitted G(r) profiles of KNN, KNN-0.06Li and KNN-0.06BNZ for\ninteratomic distance r~1-10 Å.\nR-11\nytisnetni\ndezilamroN\nobserved KNN observed KNN-0.06Li observed KNN-0.06BNZ\ncalculated R = 3.08% calculated R = 4.52% calculated R = 3.31% difference w difference w difference w\nBragg positions Pm Bragg positions P4mm Bragg positions P4mm\nBragg positions Pm Bragg positions Pm\nQ, Å-1 Q, Å-1 Q, Å-1\n6 8\n6 4 6\n4\n4\n2\n2 2\n0\n0 0\n-2 -2\n-2\n4 5 6 7 8 9 10111213141516 4 5 6 7 8 9 10 11 12 4 5 6 7 8 9 10 11 12\nytisnetni\ndezilamroN\nobserved KNN observed KNN-0.06Li observed KNN-0.06BNZ c d a if l f c e u re la n t c e e d R w = 3.28% c d a if l f c e u re la n t c e e d R w = 4.27% c d a if l f c e u re la n t c e e d R w = 2.83%\nBragg positions Pm Bragg positions P4mm Bragg positions P4mm\nBragg positions Pm Bragg positions Pm\nQ, Å-1 Q, Å-1 Q, Å-1\n9 observed 9 9\ncalculated\n6 6 6\ndifference\n3 3 3\n0 0 0\n-3 -3\n-3\n-6 -6\n-6\n2 4 6 8 10 2 4 6 8 10 2 4 6 8 10\n)r(\nG\nKNN\nR = 6.9%\nw\nr (Å)\nKNN-0.06Li KNN-0.06BNZ\nR = 7.19% R = 8.88%\nw w\nr (Å) r (Å)\n9 observed 9 9\ncalculated\n6 difference 6 6\n3 3 3\n0 0 0\n-3 -3\n-3\n-6 -6\n-6\n2 4 6 8 10 2 4 6 8 10 2 4 6 8 10\n)r(\nG\nKNN\nR = 6.14%\nw\nr (Å)\nFitting using averaged site occupancies and isotropic displacement parameters\nFitting using individual atomic positions and anisotropic displacement parameters\nKNN-0.06Li KNN-0.06BNZ\nR = 6.08% R = 7.38%\nw w\nr (Å) r (Å)\nComment 4. The manuscript uses terms like “divergent displacement” for Bi ions and “convergent\ndisplacement” for Li ions, which are unclear. Please revise these expressions to clarify the physical\nmeaning or replace them with more standard terminology.\nReply: Thanks for your insightful comment.\nHere, the “divergent displacement” and “convergent displacement” words describe the statistical\ndistribution patterns of off-center atomic displacement. These descriptors quantify the degree of\ndirectional dispersion in atomic displacement vectors, which fundamentally determines the phase\ntransition in our systems. As shown in Figure R8, the off-center displacements of Bi atoms are multi-\ndirectional and cross [110], [100], and directions. The corresponding 2D probability\ndistributions also show the multi directions a [ n 1 d 1̅ a 0 r ] e not focused. Thus, the off-center displacement of\nBi atoms is quite “divergent”. In contrast, Li atoms exhibit the off-center displacement mainly along\n[100] and directions, and their 2D probability distributions are relatively more “convergent”\nthan those o\n[1\nf̅\n0\ni\n]\natoms.\nFollowing your suggestion, we supplemented the interpretations for “divergent displacement” and\n“convergent displacement”, as shown on page 9 of the revised Manuscript.\nFigure R8. (also Figure 2e-h in the revised Manuscript) Stereographic projections of the atomic polar\ndisplacement vectors along the [001] direction for Bi and Li atoms (left). Corresponding 2D probability\ndistribution of Bi and Li atoms in KNN-0.06BNZ and KNN-0.06Li, respectively (right).\nR-12\nComment 5. On page 9, the statement “note that the spontaneous polarization directions of M and O\nphases are the same in the KNN system” appears questionable. In perovskite-type compounds, the\nspontaneous polarization in the orthorhombic phase (e.g., BaTiO ) is typically along the <110>\n3\ndirection, while in the monoclinic Pm phase (e.g., PMN-0.32PT), it is commonly along a general\n[h0l] direction. The authors should either revise this statement or provide appropriate citations.\nReply: Thanks for your insightful comment. We apologize for this inaccurate expression. We’d like to\nrespond to your question from following points.\nFirst of all, we intended to express that “For KNN system, both M and O phases were used to\ndescribe its room-temperature phase. Thus, both phases shall share similar spontaneous polarization\ndirections”. But as you commented, the spontaneous polarization in the standard orthorhombic Amm2\nphase is along the <110> direction, while in the monoclinic Pm phase, it is commonly along a general\n[h0l] direction. For KNN system near equal K/Na ratio, there is a long-standing controversy whether\nits room-temperature phase structure should be treated as orthorhombic Amm2 or monoclinic Pm.\nWhile most literature referred to the room-temperature phase as the orthorhombic Amm2 phase, recent\nstudies, including our previous work, demonstrate better data fitting with the Pm space group [see Solid\nState Sci. 2009, 11 (2), 320; Acta Mater. 2024, 266, 119691; Inorg. Chem. 2022, 61 (10), 4335; J. Appl. Phys.\n2012, 111 (10), 103503; Phys. Rev. B 2021, 103 (18), 184104]. This controversy is due to the fact that\nthe difference between the orthorhombic Amm2 and monoclinic Pm phases in KNN system is\nalmost negligible [see J. Appl. Phys. 2012, 111 (10), 103503; Solid State Sci. 2009, 11 (2), 320; Appl. Phys.\nLett. 2009, 95 (9), 091903].\nThen, the symmetry of the orthorhombic Amm2 phase of KNN system needs to be presented with\na cell doubling, and the primary perovskite unit cell exhibits a symmetry that is very close to, but not\nquite, monoclinic phase. To understand this better, the orthorhombic structure Amm2 of KNN at room\ntemperature is illustrated in Figure R9. The primary unit cell, as shown in Figure R9, has c = a > b\np p p\nand its interaxial angle β is slightly larger than 90° [see Solid State Sci. 2009, 11 (2), 320]. Therefore, the\nprimary unit cell is of monoclinic-like symmetry. In the doubled four-adjacent cell, as shown in Figure\nR9, the new structure has a′ ≠ c′ ≠ b and β′ = 90°, which satisfies the orthorhombic structure (Amm2\np\nspace group). According to the rule of crystallography, the structure should be referred to as a structure\nof higher symmetry, i.e. the orthorhombic Amm2 phase.\nR-13\n[Figure Redacted]\nFigure R9. The illustration for KNN of orthorhombic structure at room temperature. (a) The primary\nperovskite unit cell and (b) its projection along b axis. (c) Four adjacent primary perovskite unit cell\nprojections in an exaggerated manner to show the geometrical relationship, while omitting Nb and O\natoms. (a-c) are from Appl. Phys. Lett. 2007, 91 (26), 262902.\nAnd then, we collected the reported lattice parameters of orthorhombic Amm2 and monoclinic\nPm phases of KNN system, and transformed the ones of orthorhombic Amm2 phase into its primary\nperovskite unit cell (see Table R1). Clearly, the lattice parameters of monoclinic Pm phase are highly\nclose to those of the primary unit cell of orthorhombic Amm2 phase. The only difference is that a is\nequal to c in the primary unit cell of orthorhombic Amm2 phase, while a is very close to c in the\nmonoclinic Pm phase. Compared with the monoclinic Pm phase of PMN-35PT system, the difference\nbetween a and c is negligible in the monoclinic Pm phase of KNN system. Thus, for KNN system,\nalthough the spontaneous polarization direction of the monoclinic Pm phase is not precisely\nalong the <110> direction (i.e., that of orthorhombic Amm2 phase), it is very close to this direction\ndue to the negligible structure differences between these phases. Therefore, both phases exhibit\nnearly identical spontaneous polarization directions in KNN system.\nFinally, we have revised the expressions and provided appropriate citations on page 9 of the\nrevised Manuscript and on page S-37 of the revised Supplementary Information.\nTable R1. Comparison of lattice parameters between monoclinic Pm and primary perovskite unit cell\nof orthorhombic Amm2 (parameters of a and c were highlighted using the red color)\nSymmetry a (Å) b (Å) c (Å) α = γ (°) β (°) References\n3.9910(1) 3.9436(1) 4.0009(1) 90 90.31(11) This work\nPhys. Rev. B 2021,\n4.0024(2) 3.9698(2) 3.9930(2) 90 90.33(1)\n103 (18), 184104\nMonoclinic Pm in\nSolid State Sci.\nKNN system 4.0046(4) 3.9446(3) 4.0020(5) 90 90.33(5)\n2009, 11 (2), 320\nInorg. Chem. 2022,\n4.0024(2) 3.9432(1) 3.9930(2) 90 90.33(1)\n61 (10), 4335\nR-14\nPrimary perovskite unit\nPhys. Rev. 1954, 96\ncell of orthorhombic 4.0118 3.9518 4.0118 90 90.35\n(3), 581\nAmm2 in KNN system\nMonoclinic Pm in Phys. Rev. B 2002,\n4.0344 3.9873 4.0092 90 90.26\nPMN-35PT system 65 (6), 064106\n6. Density Functional Theory (DFT) Calculations\nIn Fig. 5a-c, the displacement of cations is not clearly visible. It would be helpful if the authors could\nenhance the visibility of atomic displacements (e.g., using arrows or color coding). Additionally, the\nmanuscript should specify the number of Li or Bi ions included in the supercell and how their\nconfigurations were modeled.\nReply: Thanks for your insightful comment.\nWe have revised the Figure 5a-c for showing the atomic displacements of Li and Bi atoms better,\nas shown in Figure R10. When reconstructing Figure 5a-c, we reduced the layers of supercell and\nadded the green dotted lines for better comparing the atomic displacements of Li and Bi atoms. Clearly,\nLi atoms show the larger off-center displacement than that of Bi atoms.\nFigure R10. (also Figures 5a-g in the revised Manuscript) (a-c) Relaxed crystal structure of KNN,\nR-15\nKNN-Li, and KNN-Bi supercells; (d) Nb-O bond length of KNN supercells, and Nb-O bond length\nnearest to Li and Bi atoms of KNN-Li and KNN-Bi supercells; (e-g) calculated charge density of KNN,\nKNN-Li, and KNN-Bi supercells. The green dotted lines in (a-c) are used to indicate the off-center\ndisplacement of Li and Bi atoms. The gray dotted lines in (d) mark the Nb-O bond length of KNN\nsupercells.\nTo conduct the DFT calculations, we adopted the KNN matrix as the monoclinic Pm and\ntetragonal P4mm phases. In the case of Pm phase, the supercell size is 2 × √2 × 3√2 and contains 60\natoms. In the case of P4mm phase, the supercell size is 2 × 2 × 4 and contains 80 atoms. For Li- and\nBi-doped supercells, each supercell contains one Li or Bi atom that replaced one Na atom. To make K\nand Na atoms distribute relatively equal, they were arranged alternatively along the [111] direction of\nthe primary perovskite unit cell. The detailed configurations were shown in Figure R11. Finally, we\nsupplemented the description of the DFT calculations on page 25 of the revised Manuscript and on\npage S-27 of the revised Supplementary Information.\nR-16\nFigure R11. (also Fig. S25 in the revised Supplementary Information) Supercells of KNN, KNN-\nLi, and KNN-Bi based on Pm and P4mm phases. All supercells are shown in the atomic radii mode to\nshow Li and Bi atoms better.\nR-17\nResponse to Reviewer #3:\nThe manuscript presents a comparative investigation of (K,Na)NbO -based ferroelectrics modified\n3\nwith Li and (Bi Na )ZrO , with the stated goal of uncovering atomic mechanisms responsible for\n0.5 0.5 3\nphase boundary formation and their influence on macroscopic properties. While the study combines\nmultiple experimental and theoretical approaches, several critical scientific and methodological flaws\nfundamentally undermine the validity of the authors' conclusions. These issues, detailed below, reflect\ninsufficient rigor in both the experimental design and interpretation:\nReply: Thanks for your comprehensive review. We greatly appreciate your professional insights,\nwhich have been invaluable for improving our manuscript.\nComment 1. The chemical composition of the synthesized samples should be clearly confirmed. This\nis particularly important for the Li content, as Li is a light element and its precise quantification is\ntypically challenging. The authors are encouraged to provide supporting evidence (e.g., ICP, TOF-\nSIMS, or other sensitive techniques).\nReply: Thank you for this important comment regarding chemical composition verification. We have\nconducted comprehensive compositional analysis using multiple complementary techniques to\nconfirm the successful synthesis of our target compositions.\nFor KNN-6Li ceramics, we employed two methods: the ICP-OES and TOF-SIMS measurements.\nThe ICP-OES results reveal a mass percentage (wt%=0.23) for Li element in KNN-6Li sample (see\nFigure R1), which is very close to the nominal one. All other elements (K, Na, and Nb) also show\nexcellent agreement between measured and design values, confirming successful stoichiometric\ncontrol. In addition, elements (i.e., K, Na, Li, Nb, and O) were also detected by the TOF-SIMS\nmeasurements, the mappings of which revealed the relatively homogeneous distribution of all elements\n(see Figure R2). Therefore, we did successfully dope Li into KNN matrix and fabricated KNN-xLi\nceramics.\nR-18\nFigure R1. (a) Mass percentage (wt%) of KNN-6Li sample obtained from the nominal design and the\nICP-OES measurements. (b) Obtained data and (c) calibration of ICP-OES measurements. *Note that\nICP-OES cannot give the mass percentage of oxygen. Thus, we calculate its mass fraction by\nsubtracting the mass fraction of positive ions from the total mass fraction (i.e., 100%). ICP-OES\nmeasurements were conducted and analyzed by Dr. Xi Wu in Analysis and Testing Center of Sichuan\nUniversity (https://atc.scu.edu.cn/info/1031/2366.htm).\nFigure R2. Elemental TOF-SIMS map of the surface of polished KNN-0.06Li ceramics and\ncorresponding m/Z curves of each element.\nR-19\nFor KNN-6BNZ sample, we analyzed the chemical composition by using field emission scanning\nelectron microscopy (FE-SEM, JSM-7500, Japan) (see Figure R3). All elements reveal a relatively\nhomogeneous distribution. We analyzed the atomic percentage (at%) on two different areas. For each\nelement, the measured values were close to the nominal ones. Therefore, these observations well\nproved that we did prepare KNN-6Li and KNN-6BNZ ceramics as we designed.\nThe results from ICP-OES, TOF-SIMS, and SEM-EDS measurement confirm successful\nsynthesis of target compositions with precise stoichiometric control. These characterization results\nhave been incorporated into the revised Supplementary Information (see Figs. S1-S3).\nFigure R3. (also Fig. S2 in the revised Supplementary Information) SEM-EDS elemental mappings\nand corresponding atomic percentage analysis of KNN-0.06BNZ ceramics.\nComment 2. In the KNN structure, Li⁺ ions can occupy either the A-site or B-site. The manuscript does\nnot provide sufficient evidence to distinguish between these possibilities. How did the authors\ndetermine the preferred site(s) of substitution and quantify the distribution of Li between the A- and\nR-20\nB-sites?\nReply: Thanks for this important question regarding Li+ site occupancy. We have investigated this\nissue and provide evidence that Li+ ions prefer to enter A-site rather than B-site from the following\nperspectives.\n(1) In literatures, Li+ is highly likely to substitute for A-site K+ and Na+ rather than B-site\nNb5+ in the (K, Na)NbO system since Li+ (+1) perfectly matches K+/Na+ (+1) valence states. When\n3\ndesigning the formula, we intentionally reduced the K and Na elements in A-site below the\nstoichiometric ratio and used Li element to supplement the reduced portion, namely (K Na )\n0.48 0.52 1-\nLi NbO (same as (1-x)(K Na )NbO -xLiNbO in the original manuscript). We guess that it was\nx x 3 0.48 0.52 3 3\nthe chemical formula that made you think that we mixed (K Na )NbO with LiNbO to produce\n0.48 0.52 3 3\nKNN-Li. The fact is that we mixed the raw materials of K CO , Na CO , Nb O , and Li CO directly\n2 3 2 3 2 5 2 3\nand then fabricated KNN-Li ceramics according to the conventional solid-state reaction method. Note\nthat the chemical formula of (K Na ) Li NbO or (1-x)(K Na )NbO -xLiNbO has been\n0.48 0.52 1-x x 3 0.48 0.52 3 3\nwidely reported in previous references [see Appl. Phys. Lett. 2004, 85 (18), 4121; Jpn. J. Appl. Phys. 2005,\n44 (8R), 6136; Ceram. Int. 2023, 49 (7), 11101; Acta Mater. 2024, 266, 119691]. Undoubtedly, all these\nreferences believed that “Li mainly replaces Na and K in the A sites of ABO perovskite structure (i.e.\n3\nform a solid solution)” [see Appl. Phys. Lett. 2004, 85 (18), 4121].\n(2) X-ray diffraction analysis provides definitive structural proof. Based on Bragg's Law\n(2d·sinθ=n·λ) and ionic radii considerations, if Li+ ions enter B-site to replace Nb5+ ions, the XRD\npatterns should shift to lower angles because of the larger ionic radius of Li+ than Nb5+ (see Table R1)\n(see Acta Crystallogr. Sect. A: Cryst. Phys. Diffr. Theor. Gen. Crystallogr. 1976, 32 (5), 751; J. Mater. Chem. C\n2019, 7 (15), 4284). On the contrary, the XRD patterns will shift to higher angles if Li+ ions enter A-site\nbecause of the smaller ionic radius of Li+ than K+/Na+ (see Table R1). We collected the XRD patterns\nof Li-doped KNN ceramics from references and also measured XRD patterns of (K Na )\n0.48 0.52 1-\nLi NbO (x=0.075-0.10) ceramics prepared by ourselves (see Figure R4). Since these XRD patterns\nx x 3\nwere measured at room temperature, we need to compare the XRD patterns of compositions with a\nsingle phase (i.e., an O phase at x=0-0.04 and a T phase at x>0.07) in order to exclude the effect of\nroom-temperature O-T phase coexistence on the XRD patterns.\n For compositions with a single O phase (i.e., x=0-0.04), both references show that the XRD\npatterns (covering both low and high angle ranges) significantly shift to higher angles and\nR-21\nthe corresponding lattice parameters and unit cell volume also exhibit a decrease (see Figure\nR4 (a, b)), indicating that Li+ ions should enter A-site.\n For compositions with a single T phase (i.e., x>0.07), both reference and our results show\nthat (200) peak shifts to lower angles slightly, but (002) peak shifts to higher angles\nsignificantly, which is proved by the greater increase in lattice parameter c than the decrease\nin the lattice parameter a/b (see Figure R4(a)) {also the greater increase in the center position\nof (002) peak than the decrease in the center position of (200) peak (see Figure R4(c)). The\nslight decrease in a/b and the significant increase in c are caused by the increased\ntetragonality (c/a) in KNN-based ceramics with a high Li content [see Ceram. Int. 2012, 38 (1),\n147]. Overall, the XRD patterns moved towards high angles, manifesting that Li+ ions still\nentered A-site of KNN ceramics even at the high doping content because of the deliberately\ndesigned chemical formula of Li (Na K ) NbO .\nx 0.5 0.5 1-x 3\nIn addition, similar phenomenon was also observed in Li-doped KNbO ceramics. As expected,\n3\nreplacing K+ with Li+ shifted the XRD patterns to higher angles and reduced the lattice parameters\nbecause of the smaller ionic radius of Li+ than K+ (see Figure R5). Therefore, all these observations\nunambiguously indicate that Li+ ions enter A-site rather than B-site in KNN or KN ceramics.\n(3) Dielectric and ferroelectric properties rule out B-site occupation. The increased Cuire\ntemperature (T ) and reduced diffusion factor (γ) preclude the possibility of Li+ ions entering B-site.\nC\nAs you pointed out, Li+ ions and Nb5+ ions differ by four valences. If Li+ ions replace Nb5+ ions, it will\ncause considerable local random electric fields and stress fields, which will inevitably significantly\ndisrupt the long-range ferroelectric order, thereby leading to the appearance of dielectric relaxation (\nor dispersion) and a decrease in T [see J. Am. Ceram. Soc. 2012, 95 (1), 1]. However, this speculation is\nC\ncompletely contrary to the facts observed by others and ourselves, that is, the T increased with\nC\nincreasing Li+ content [see Appl. Phys. Lett. 2004, 85 (18), 4121; Curr. Appl. Phys. 2011, 11 (3), S149]. In\naddition, doping Li+ was also reported to reduce the dc conductivity of Li (Na K ) NbO ceramics,\nx 0.5 0.5 1-x 3\nas shown in Figure R6 [see Curr. Appl. Phys. 2011, 11 (3), S149]. This clearly does not correspond to the\nsituation where Li+ ions replace the B-site Nb5+, because Li+ ions occupying the B-site would produce\nsignificant oxygen vacancies that increase the conductivity.\n(4) We searched the possibility of Li+ ions entering B-site of ABO -type perovskite\n3\npiezoceramics and found that some references reported that Li+ ions might entered B-site of BaTiO\n(BTO) ceramics [see J. Am. Ceram. Soc. 2018, 101 (8), 3597; Mater. Chem. Phys. 2019, 231, 173; J. Mater.\nSci: Mater. Electron. 2019, 30 (7), 6556]. However, this also depends on the specific circumstances, as\nsummarized in Figure R7.\n For type-A BTO ceramics (i.e., (Ba Li )TiO ), Li+ ions were deliberately designed to\n1-x x 3-x/2\nreplace A-site Ba2+ ions, which resulted in the reduced volume because of the smaller ionic\nradius of Li+ than Ba2+. Note that such substitution persisted even at x=7%, and is analogous\nto the situation of our [(K Na ) Li ]NbO ceramics.\n0.48 0.52 1-x x 3\n For type-B BTO ceramics (i.e., (Ba(Ti Li )O ), the volume increased because of\n1-x x 3-3x/2\ndeliberately replacing Ti4+ with Li+.\n For-type C (i.e., x/2 Li CO +BaTiO ), the volume first decreased and then increased because\n2 3 3\nof directly adding Li CO into BaTiO ceramics without deliberately design non-\n2 3 3\nstoichiometric ratio at neither A-site nor B-site.\nIn our work and previous references, Li+ ions were deliberately designed to replace A-site of KNN\nceramics (i.e., [(K, Na) Li ]NbO ), which is the same as the situation of type-A BTO ceramics. In\n1-x x 3\nparticular, the Li content needed for constructing room-temperature O-T phase boundary is only 6%\nmolar ratio at A-site (i.e., [(K, Na) Li ]NbO ). That is the reason why we and other researchers\n0.94 0.06 3\nreported observations including the decreased volume, the XRD patterns shifting to higher angles,\nand the reduced dc conductivity.\nMultiple evidence - chemical analysis, structural analysis, property measurement, and charge\nbalance consideration- convinced us that Li+ ions did enter the A-site in our KNN ceramics. To make\nthis point clearer, we have added relevant discussion in the text, as shown on pages 5 and 20 of revised\nManuscript and in Figs. S1-S3 of revised Supplementary Information.\nTable R1. Ionic radii of Li+, Na+, K+, and Nb5+ ions. (CN is the coordination number)*\nIons Li+ K+ Na+ Nb5+\nIonic radius 0.76 Å (CN=6) 1.38 Å (CN=6) 1.02Å (CN=6) 0.64 Å (CN=6)\n1.25 Å (CN=12) 1.64 Å (CN=12) 1.39 Å (CN=12)\n* Note that the CN of A-site cation is 12 [see J. Mater. Chem. C 2019, 7 (15), 4284].\nR-23\n[Figure Redacted]\nFigure R4. (a) Enlarged XRD patterns of [Li (Na K ) ]NbO ceramics with 2θ=45-50° and\nx 0.5 0.5 1-x 3\ncorresponding lattice parameters [from Appl. Phys. Lett. 2004, 85 (18), 4121]; (b) Enlarged XRD patterns\nand unit cell volume of Li (Na K ) NbO (KNN-xLi, x=0, 0.02, and 0.04) ceramics with 2θ=83.8-\nx 0.5 0.5 1-x 3\n84.4° [from Acta Mater. 2024, 266, 119691]; (c) Enlarged XRD patterns of Li (Na K ) NbO\nx 0.5 0.5 1-x 3\n(x=0.075, 0.08, and 0.10) ceramics with 2θ=45-46° and corresponding center position of (200) and\n(002) peaks.\n[Figure Redacted]\nFigure R5. Enlarged XRD patterns of KNbO and (K Li )NbO ceramics and fitting results [from\n3 0.95 0.05 3\nnpj Comput. Mater. 2025, 11 (1), 92]\n[Figure Redacted]\nFigure R6. dc conductivity of KNN-xLN ceramics [from Curr. Appl. Phys. 2011, 11 (3), S149]\n[Figure Redacted]\nFigure R7. Three types of Li-doped BTO ceramics [from J. Am. Ceram. Soc. 2018, 101 (8), 3597]\nComment 3. The substitution of Bi3+ into the KNN lattice introduces valence imbalance. The authors\nshould elaborate on the mechanism of charge compensation. Specifically, what is the role of A-site\nvacancies in maintaining charge neutrality?\nReply: Thanks for your insightful comment. We’d like to answer your question from the following\npoints.\nFirst, the chemical formula KNN-yBNZ is charge neutrality overall. As shown in Figure R8, (1-\ny)(K Na )NbO -y(Bi Na )ZrO can be written as “[(A Bi )][Nb Zr ]O ”, where A\n0.48 0.52 3 0.5 0.5 3 1-0.5y 0.5y 1-y y 3\ndenotes the A-site K+/Na+ ions. In this case, considering the valance of A+, Bi3+, Nb4+, Zr4+, and O2-,\nR-24\nthe charge neutrality of KNN-yBNZ is thus remained overall.\n(1-y)(K Na )NbO -y(Bi Na )ZrO\n0.48 0.52 3 0.5 0.5 3\nStep 1\n[(K Na ) (Bi Na ) ][Nb Zr ]O\n0.48 0.52 1-y 0.5 0.5 y 1-y y 3\nStep 2\n[(K Na )(Bi )][Nb Zr ]O\n0.48(1-y) 0.52(1-y)+0.5y 0.5y 1-y y 3\nStep 3\n[(K Na )(Bi )][Nb Zr ]O\n0.48(1-y) 0.52-0.02y 0.5y 1-y y 3\nStep 4\n[(A Bi )][Nb Zr ]O\n1-0.5y 0.5y 1-y y 3\nTotal valence\nvalue of +1×(1-0.5y)+3×(0.5y)+5×(1-y) +4×(y) = 6\nA-& B-site ions\nFigure R8. The chemical formula of KNN-yBNZ and corresponding charge neutrality. Note that the\nred and blue colors denote the A-site and B-site ions, respectively. For step 4, since K+ and Na+ have\nthe same valance and occupy at the A-site, we thus simplify then as the “A” site.\nSecond, let’s understand this from the defect chemistry. As you commented, if only considering\nBi3+ ions entering A-site of KNN ceramics, the A-site vacancies should be generated according to the\nfollowing equation [see Nat. Commun. 2021, 12 (1), 881]:\n(1)\n𝐾𝑁𝑁\n•• ′\nIf only considering Zr4+ ions𝐵 𝑖e2n𝑂te3ri n⎯g B2-𝐵si𝑖t𝐾e/ 𝑁o𝑎f +KN4𝑉N𝐾 /c𝑁e𝑎ra+m3ic𝑂s𝑂, the oxygen vacancies should be\ngenerated according to the following equation [see Nat. Commun. 2021, 12 (1), 881]:\n(2)\n𝐾𝑁𝑁\n′ ••\nIn the case of (Bi\n0.5\nNa\n0.5\n)ZrO\n3\n(B2N𝑍Z𝑟𝑂) 2do p⎯ an2t, 𝑍th𝑟𝑁e 𝑏m+ol𝑉e𝑂 ra+tio4 o𝑂f𝑂 Bi:Zr is 1:2. Therefore, when doping\nBNZ into KNN ceramics, the above two equations should occur simultaneously in the following\nequation:\n𝐾𝑁𝑁\n•• ′ ′ ••\nClearly, the con0c.o5m𝐵i𝑖t2a𝑂n3t A+-s2i𝑍te𝑟 v𝑂a2c a⎯n cie𝐵s𝑖 a𝐾n/𝑁d 𝑎o+xy2g𝑍en𝑟 𝑁v𝑏ac+an2c𝑉i𝐾es/ 𝑁o𝑎cc+ur𝑉 t𝑂o r+em5a.5in𝑂 𝑂th (e3 c)harge neutrality\noverall in KNN-yBNZ ceramics, as marked by the red color in the Equation (3).\nFinally, we have supplemented the mechanism of charge compensation, as shown on page 20 of\nrevised Manuscript.\nR-25\nComment 4. In the case of Li substitution, particularly if Li occupies the B-site, oxygen vacancies may\nform to maintain charge balance. This aspect should be discussed in greater depth, ideally with\nsupporting evidence from techniques such as XPS or other methods capable of evaluating defect\nchemistry or charge compensation.\nReply: Thanks for your insightful comment regarding charge compensation mechanisms in KNN-\nbased ceramics\nWe have supplemented the O 1s XPS spectra analysis for KNN, KNN-0.06Li, and KNN-0.06BNZ\nceramics, as shown in Figure R9. All ceramics show the similar O 1s XPS spectra. The fitting results\nreveal three peaks, which correspond to the lattice oxygen (O ), oxygen vacancy (O ), and adsorbed\nL V\noxygen (O ), respectively. KNN-0.06Li ceramics exhibit the lowest O content, and KNN and KNN-\nads V\n0.06BNZ ceramics show a comparable O content. The lowest O content of KNN-0.06Li ceramics\nV V\nmay be related to its low sintering temperature of ~1040 ℃, lower than that of KNN (~1100 ℃) and\nKNN-0.06Li (~1080 ℃) ceramics. As you commented, if Li occupies the B-site, oxygen vacancies\nmay form to maintain charge balance. Therefore, the XPS results manifest that Li occupies the A-site\ninstead of B-site. Finally, we have added the O 1s XPS spectra analysis, as shown on page S-4 of\nrevised Supplementary Information.\nR-26\nraw\nO L\nO v\nO\nads\nfitting\nKNN-0.06BNZ\nO =52.46%\nL\nO =32.36%\nv\nO =15.18%\nads\n540 535 530 525\nFigure R9. (also Fig. S3 in the revised Supplementary Information) O 1s XPS spectra analysis of\nKNN, KNN-0.06Li, and KNN-0.06BNZ ceramics.\nComment 5. The experimental results suggest the coexistence of monoclinic (Pm) and tetragonal\n(P4mm) phases. Therefore, DFT calculations should explicitly consider both symmetries to ensure the\naccuracy and consistency of the theoretical predictions.\nReply: Thanks for your insightful comment regarding the theoretical treatment of phase coexistence\nWhile DFT calculations cannot directly model coexisting phases within a single supercell,\nwe\nhave supplemented the DFT calculations for both symmetries (i.e., monoclinic (Pm) and tetragonal\n(P4mm) separately, as shown in Figures R10-R11. Our DFT calculations reveal symmetry-\nindependent dopant behavior: Li atoms exhibit larger off-center displacements compared to Bi atoms,\nregardless of matrix symmetry. More importantly, when setting the matrix symmetry as tetragonal\n(P4mm), Li atoms show a more significant off-center displacement along the c axis (i.e., the\npolarization direction of tetragonal phase) leading to the nearest Nb atoms also shifting more along the\nc axis. The synergetic off-center displacements of Li and Nb atoms result in the observed Pm to P4mm\nR-27\n).u\n.a(\nytisnetnI\nKNN-0.06Li\nO\nO =55.02% L\nL\nO =29.00%\nv\nO ads =15.97% O\nv\nO\nads\n).u\n.a(\nytisnetnI\nKNN O\nL\nO =55.31% L\nO =33.17%\nv\nO\nO =11.52% v\nads\nO\nads\n).u\n.a(\nytisnetnI\nO\nL\nO\nv\nO\nads\nBinding energy (eV)\ntransition and large tetragonality (c/a) in KNN-Li ceramics. Thus, we have supplemented results and\ndiscussion, as shown on page 15 of the revised Manuscript and on pages S27-S28 of the revised\nSupplementary Information.\nFigure R10. (also Figures 5a-g in the revised Manuscript) DFT calculations by setting the matrix\nsymmetry as monoclinic (Pm). (a-c) Relaxed crystal structure of KNN, KNN-Li, and KNN-Bi\nsupercells; (d) Nb-O bond length of KNN supercells, and Nb-O bond length nearest to Li and Bi atoms\nof KNN-Li and KNN-Bi supercells; (e-g) calculated charge density of KNN, KNN-Li, and KNN-Bi\nsupercells. The green dotted lines in (a-c) are used to indicate the off-center\ndisplacement of Li and Bi atoms. The gray dotted lines in (d) mark the Nb-O bond length of KNN\nsupercells.\nR-28\nFigure R11. (also Fig. S26 in the revised Supplementary Information) DFT calculations by setting\nthe matrix symmetry as tetragonal (P4mm). (a-c) Relaxed crystal structure of KNN, KNN-Li, and\nKNN-Bi supercells; (d) Nb-O bond length of KNN supercells, and Nb-O bond length nearest to Li and\nBi atoms of KNN-Li and KNN-Bi supercells; (e-g) calculated charge density of KNN, KNN-Li, and\nKNN-Bi supercells. The green dotted lines in (a-c) are used to indicate the off-center\ndisplacement of Li and Bi atoms. The gray dotted lines in (d) mark the Nb-O bond length of KNN\nsupercells.\nComment 6. In the experimental section, 0.2 mol% Fe O was added to promote sintering in KNN-\n2 3\nyBNT ceramics, whereas KNN-Li samples were synthesized without Fe O . Given that Fe3+ ions can\n2 3\ninfluence electrical and structural properties even in small amounts, how did the authors confirm that\nFe3+ was not unintentionally introduced into the KNN-Li samples, or that its presence did not affect\nthe observed properties?\nReply: Thanks for your insightful comment. We’d like to answer your question from the following\npoints.\nFirst, the absence of 0.2 mol% Fe O in KNN-Li ceramics stems from the intrinsic sintering\n2 3\nenhancement properties of Li. Li CO with its low melting point (~720 ℃) act as inherent sintering\n2 3\nR-29\naid that promotes sintering and lowers the sintering temperature (T ) in addition to shifting the phase\ns\ntransition temperature. Actually, there have already references that used Li CO to reduce T of both\n2 3 s\nlead-based and lead-free ceramics [see Ceram. Int. 2015, 41 (2, Part B), 2774; J. Alloys. Compd. 2018, 735,\n386; Mater. Res. Bull. 2024, 173, 112705]. In particular, it was reported that KNN-Li ceramics can be well\nsintered at the T of 1040-1100 ℃ [see Appl. Phys. Lett. 2004, 85 (18), 4121; J. Appl. Phys. 2012, 111 (10),\ns\n103503; Curr. Appl. Phys. 2011, 11 (3), S149]. Conversely, BNZ doping presents sintering difficulties\nrequiring T of 1150-1230℃ without additives, causing substantial alkali metal (e.g., K and Na) loss\ns\nand compromising stoichiometry [see J. Am. Ceram. Soc. 2017, 100 (2), 627]. Thus, we chose 0.2 mol%\nFe O as the sintering aid and reduced the T to 1080 ℃, analogous to situations of other references\n2 3 s\nthat utilized various sintering aids (e.g., Fe O and MnO ) [see J. Am. Ceram. Soc. 2015, 98 (6), 1698;\n2 3 2\nEnergy Environ. Sci. 2018, 11 (12), 3531; J. Phys. Chem. Solids 2009, 70 (3-4), 750].\nThen, we compared the phase transition temperature (i.e., T and T ) and d values of KNN-\nO-T C 33\nBNZ ceramics prepared by our and other groups. As shown in Figure R12, our KNN-yBNZ-Fe\nceramics show very similar change tendency on T and T compared with other KNN-xBNZ\nO-T C\nceramics, further indicating that Fe3+ ions hardly affected the phase transition temperature. More\nimportantly, our KNN-yBNZ-Fe ceramics show comparable d values but lower T , compared with\n33 s\nother KNN-BNZ ceramics (see Table R2). Therefore, these results demonstrate that the variations of\nphase transition temperature and electrical properties in our KNN-yBNZ-Fe ceramics were mainly\nattributed to the addition of BNZ, and 0.2 mol% Fe O mainly acted as the sintering aid.\n2 3\n[Figure Redacted]\nFigure R12. (also Fig. S32 in the revised Supplementary Information) T and T values of (1-\nO-T C\nx)(K Na )NbO -x(Bi Na )ZrO (abbreviated as KNN-xBNZ) and (1-y)(K Na )NbO -\n0.4 0.6 3 0.5 0.5 3 0.48 0.52 3\ny(Bi Na )ZrO -0.2mol%Fe O (abbreviated as KNN-yBNZ-Fe) ceramics. Data of KNN-xBNZ were\n0.5 0.5 3 2 3\nextracted from J. Am. Ceram. Soc. 2017, 100 (2), 627, and data of KNN-yBNZ-Fe were from this work.\nTable R2. d and T values of three KNN-BNZ ceramics.\n33 s\nComposition d (pC/N) T (℃) References\n33 s\n0.955(K Na )- 320 1150-1230 J. Am. Ceram. Soc.\n0.44 0.56\nR-30\n0.045(Bi Na )ZrO 2017, 100 (2), 627\n0.5 0.5 3\n0.95(K Na )- J. Am. Ceram. Soc.\n0.5 0.5\n360 1100-1120\n0.05(Bi Na )ZrO 2014, 97 (3), 688\n0.5 0.5 3\n0.94(K Na )-\n0.48 0.52\n360 1080 This work\n0.06(Bi Na )ZrO -0.2molFe O\n0.5 0.5 3 2 3\nAs for your comment that “how did the authors confirm that Fe3+ was not unintentionally\nintroduced into the KNN-Li samples”, we guaranteed that Fe3+ was not introduced into KNN-Li\nsamples. This is because we did not add Fe O raw material actually and prepared KNN-Li ceramics\n2 3\nby using totally different tools including milling jars and muffle furnace.\nFinally, we have emphasized this point based on your comment in the revised Manuscript (see\npages 20-21) and Supplementary Information (see page S-34) to address your valuable concern.\nR-31"}, {"round": 2, "reviewer_comments": {"reviewer_1": "All remarks and suggestions have been addresed. As a consequence the manuscript can be accepted in the present form", "reviewer_2": "Thank you for submitting the revised manuscript. I have carefully read your responses regarding the significance and novelty of the work. However, I still believe that the current manuscript is more appropriate for a specialized materials science journal rather than Nature Communications. The readership of Nature Communications is likely already well aware of the technological relevance of lead-free piezoelectrics. While the proposed off-centering mechanism for specific elements in KNN may be a new insight within this system, similar mechanisms have already been reported in other lead-free piezoelectric systems (e.g., Commun. Mater., 1:71, 2020). Furthermore, it remains unclear whether the proposed design principles are broadly applicable to other material systems.\nBelow are my detailed responses to your replies to my previous comments:\n________________________________________\nOn Grain Size:\nReplacing the average grain size with the median does not appear to be a suitable solution. At minimum, systems exhibiting abnormal grain growth should be treated separately from those showing normal grain growth, as their growth mechanisms are fundamentally different and cannot be evaluated using the same statistical treatment.\nOn Relative Density:\nThank you for providing detailed information regarding the measurement methodology. However, I remain concerned about the accuracy of the reported relative densities. Although the method employed is commonly used, I have similar reservations about the reliability of density values reported in other literature. For instance, the area of the pores visible as dark contrasts in Fig. R3 appears inconsistent with the reported porosity values of only 0.2–1.4%. A plausible explanation for this discrepancy could be the presence of closed pores, which are not filled with water during measurement and thus are not accounted for in Archimedes’ method.\nNeutron Diffraction and PDF Analysis:\nThank you for addressing my concerns and improving the analysis in this section. The revisions have clarified the interpretation.\nOn “Convergent” and “Divergent” Displacements:\nThank you for the added explanation. The revised text is clearer in conveying the intended meaning.\nOn Crystallography in M and O Phases:\nThank you for your response. The clarification on polarization directions and references provided are appropriate, and I find the explanation satisfactory.\nDFT Calculations:\nThank you for the revisions and improvements made to this section. The clarification on atomic configurations has enhanced the credibility of the computational analysis.", "reviewer_3": "The manuscript has been carefully revised according to the reviewers’ comments. I find the revisions satisfactory and recommend acceptance in its present form."}, "author_response": "Response to Reviewers\n(Manuscript no: NCOMMS-25-48604)\nResponse to Reviewer #1:\nAll remarks and suggestions have been addressed. As a consequence the manuscript can be accepted\nin the present form.\nReply: Thanks for your helpful suggestions as well as your kind recognition and recommendation of\nour work.\nR-1\nResponse to Reviewer #2:\nThank you for submitting the revised manuscript. I have carefully read your responses regarding the\nsignificance and novelty of the work. However, I still believe that the current manuscript is more\nappropriate for a specialized materials science journal rather than Nature Communications. The\nreadership of Nature Communications is likely already well aware of the technological relevance of\nlead-free piezoelectrics. While the proposed off-centering mechanism for specific elements in KNN\nmay be a new insight within this system, similar mechanisms have already been reported in other lead-\nfree piezoelectric systems (e.g., Commun. Mater., 1:71, 2020). Furthermore, it remains unclear\nwhether the proposed design principles are broadly applicable to other material systems.\nBelow are my detailed responses to your replies to my previous comments:\nReply: We sincerely appreciate your constructive suggestions, which have enhanced our manuscripts\nclarity. We understand your concerns regarding novelty and broader applicability, and we provide the\nfollowing clarification.\nA. Fundamental scientific breakthrough beyond simple off-center displacement\n1. Critical importance of room-temperature phase boundaries in ferroelectrics\nThe piezoelectric performance of undoped ferroelectric ceramics is intrinsically limited, with\npiezoelectric coefficient (d ) typically below 200 pC/N for pristine (K, Na)NbO , BaTiO , and PbTiO\n33 3 3 3\nceramics. To improve d values, researchers have strategically employed ion doping to engineer room-\n33\ntemperature phase boundaries where multiple phases coexist, thereby achieving the improvement of\ndielectric and piezoelectric properties. However, despite decades of research, critical knowledge gaps\npersist regarding atomic-scale mechanisms by which different dopants construct these phase\nboundaries and their impact on electrical performance. This lack of fundamental understanding has\nconfined the development of high-performance lead-free piezoelectric to empirical trial-and-error\napproaches, hindering the design of advanced piezoelectrics.\n2. Convergent vs divergent atomic displacement\nThe significance of our work lies in decoding the atomic-scale behavior, by which Bi3+ and Li+\nconstruct the identical M-T phase boundary through different mechanisms, directly linking atomic\nbehavior to mesoscopic ferroelectric domains and macroscopic piezoelectric performance. Our neutron\nPDF analysis reveals the experimental evidence of directional dopant displacement patterns:\nR-2\nconvergent displacement of Li+ and divergent displacement of Bi3+. These different displacements\ndrive distinct phase construction pathways, namely Li+ induces a combined displacive & order-\ndisorder transition, while Bi3+ triggers a predominant order-disorder transition. This mechanistic\ndifferentiation establishes the complete atomicmesoscopicmacroscopic correlation chain,\nexplaining why identical M-T phase boundaries yield dramatically different electrical properties. To\nour knowledge, such mechanistic insight has not been previously reported.\n3. Fundamental distinctions: beyond off-center displacement you mentioned\nWhile we acknowledge that ionic off-center displacements have been observed in ferroelectric\nsystems, our work represents a new paradigm in mechanism understanding. We compared our findings\nwith the literature you mentioned (Commun. Mater., 1:71, 2020) to highlight the essential differences\ndespite both involving off-center displacement, as listed in Table R1. Clearly, the reference you\nmentioned observed off-center displacements as structural phenomena, while our work deciphers how\ndifferent displacement directionalities construct identical phase boundaries through different atomic\nmechanisms. Our work represents comprehensive atomic-level explanation for dopant-induced phase\nboundary construction in room-temperature piezoelectric systems, providing a fundamental\nunderstanding for lead-free piezoelectric.\nTable R1. The differences between our work and the literature you mentioned.\nReference you mentioned (Commun.\nOur work\nMater., 1:71, 2020)\nMaterial 1. Both KNN-BNZ and KNN-Li possessed the identical 1. 0.3BT-0.1BMT-0.6BF only exhibited\nsystem room-temperature M-T phase boundary and thus exhibited a pseudo-cubic phase below T , without\nC\nsituation the improved d values. However, KNN-BNZ showed a a room-temperature phase boundary or\n33\nmuch higher d than KNN-Li, indicating other underlying ferroelectric domains. Thus, it only\n33\nphysical mechanisms beyond the M-T phase boundary. exhibited the electrostriction but no d\n33\nvalues.\n2. Bi3+ and Li+ ions are doping ions. 2. Bi3+ ions are host ions.\nTraits of 1. The proposed off-center displacement of Bi3+ and Li+ ions 1. The proposed off-center displacement\nthe fundamentally revealed the origin of the formation of M-T of Bi3+ ions was used to explain the\nproposed phase boundaries in KNN system, and successfully explained electrostriction of the 0.3BT-0.1BMT-\nmechanism the significantly different mesoscopic ferroelectric domains 0.6BF system under high electric fields,\nand macroscopic piezoelectric properties beyond the rather than d values.\n33\ncanonical cognition that generally attributed the enhanced\npiezoelectricity to the phase boundary but could not explain\nthe different electrical properties observed in similar phase\nboundaries.\nR-3\n2. The neutron total scattering endowed us to indicate the 2. The synchrotron radiation diffraction\ndirections and the degree of divergence of Bi3+ and Li+ ions’ only indicated the directions of Bi3+ ions’\noff-center displacement. As proved in our work, the different off-center displacement, but it did not\ndegrees of divergence of Bi3+ and Li+ ions were the reflect divergence or convergence of the\nfundamental cause of the performance differences. displacement.\n3. The proposed off-center displacement of Bi3+ and Li+ ions 3. The proposed off-center displacement\nwas closely associated with mesoscopic domain structures. of Bi3+ ions was not associated with\nmesoscopic domain structures.\nB. The proposed design principles are broadly applicable to other material systems\nYour question regarding broader applicability is crucial for accessing the significance of our work.\nThrough literature research, we have found some potential evidence. Doping Bi3+ and Li+ ions showed\nthe similar effects on BTO ceramics. Specifically, even though the decreased T after doping Li+ and\nO-T\nBi3+ ions in BTO ceramics, doping Li+ ions showed the unchanged (or slightly increased) T and the\nC\nenhanced tetragonality (c/a) [analogous to the situation of KNN-xLi of our work] (see Figure R1(a-\nc)). Conversely, doping Bi3+ ions significantly sacrificed both T and tetragonality (c/a) and\nC\nfragmented the ferroelectric domains [analogous to the situation of KNN-yBNZ of our work] (see\nFigures R1(d-f) and R2). Therefore, the consistency between KNN and BTO system confirm the\nbroad application of our proposed mechanism, although further experiments are necessary for\nestablishing our framework as a universal design principle for ferroelectric optimization.\n[Figure Redacted]\nFigure R1. Effects of doping (a-c) Li and (d-f) Bi ions on the lattice parameters and phase transition\ntemperature points of BTO ceramic system. Data of (a, b) was derived from J. Eur. Ceram. Soc. 2013, 33\n(5), 1009; data of (c) was derived from J. Mater. Chem. A 2019, 7 (29), 17366; data of (d, e) was derived from\nJ. Am. Ceram. Soc. 2009, 92 (1), 110; data of (f) was derived from Phys. Status Solidi A 2001, 184 (2), 459. Doing\nLi+ ions first increased the tetragonality (c/a) and then decreased it. While doping Li+ ions and\n(Li Ho )2+ made the T unchanged or slightly increased. On the contrary, doping Bi3+ ions and\n0.5 0.5 C\nBiScO significantly sacrificed both T and tetragonality (c/a).\nR-4\n[Figure Redacted]\nFigure R2. Temperature-dependent dielectric properties of (1-x)BaTiO -xBiScO ceramics. Data of (a,\n3 3\nb) was derived from J. Am. Ceram. Soc. 2009, 92 (1), 110. Although the dielectric anomalies associated with\nT and T were difficult to distinguish at x=0.005 because of the significantly disrupted long-range\nR-O O-T\nferroelectric order, the anomalies were located at a temperature below 0 ℃ that was lower than T\nO-T\nof x=0. Thus, doping BiScO reduced T of BTO ceramics.\nFinally, in response to your insight comments, we have addressed two critical aspects: the\ndistinctions between our work and the referenced literature, and the broader applicability of our\nframework, as shown on Page 19 of the revised Manuscript.\nComment 1. On Grain Size:\nReplacing the average grain size with the median does not appear to be a suitable solution. At minimum,\nsystems exhibiting abnormal grain growth should be treated separately from those showing normal\ngrain growth, as their growth mechanisms are fundamentally different and cannot be evaluated using\nthe same statistical treatment.\nReply: Thanks for your insightful comment.\nWe have revised the statistical analysis of grain size distribution. Since KNN and KNN-\n0.045BNZ samples contained the significantly abnormal grain growth, we have treated the abnormal\ngrain growth and the normal grain growth separately for both samples. Please note that the abnormally\nlarge grains in both samples were relatively few and did not have a decisive impact on the statistical\nanalysis of grain size. This was proved by exampling the KNN sample (see Figure R1). Finally, based\non your comment, we only counted the normal grain growth and appropriately mentioned the cases of\nextremely large grains in the text (see Figure R2). The related revisions can be found in Fig. 12 of the\nrevised Supporting Information.\nR-5\nBoth abnormal and normal grain growth Only normal grain growth\n40\n100 30\n30 80\n20 60\n20\n40\n10 10\n20\n0 0 0\n0 2 4 6 8 10 0.0 0.4 0.8 1.2 1.6 2.0\nFigure R1. Grain size distributions of polished and thermally-etched KNN sample when considering\nboth abnormal and normal grain growth (left) and only normal grain growth (right). Because the\nabnormally large grains were relatively few, the obtained d values remained almost unchanged.\ncorresponding grain size distributions of polished and thermally-etched (a) KNN, (b) KNN-0.06Li, (c)\nKNN-0.06BNZ, and (d) KNN-0.045BNZ ceramics. Since the much larger grains in KNN-0.06Li, we\nused a FE-SEM image with a low magnification to make more grains being considered. Note that the\ngrain size distribution in (d) corresponds to the left FE-SEM image. The right FE-SEM image in (d)\nR-6\n)%(\n.qerf\nevitaleR\n100\n80\n60\n40\n20\n)%(\n.qerf\nevitalumuC\nd =0.78 μm\n50\n)%(\n.qerf\nevitaleR\nGrain size (μm)\n)%(\n.qerf\nevitalumuC\nd =0.77 μm\n50\nshows the rarely abnormally large grain of KNN-0.045BNZ ceramics. The yellow arrows in (a, d)\nmark the size of rarely abnormally large grains. These abnormally large grains were relatively few and\ndid not have a decisive impact on the statistical analysis of grain size.\nComment 2. On Relative Density:\nThank you for providing detailed information regarding the measurement methodology. However, I\nremain concerned about the accuracy of the reported relative densities. Although the method employed\nis commonly used, I have similar reservations about the reliability of density values reported in other\nliterature. For instance, the area of the pores visible as dark contrasts in Fig. R3 appears inconsistent\nwith the reported porosity values of only 0.2-1.4%. A plausible explanation for this discrepancy could\nbe the presence of closed pores, which are not filled with water during measurement and thus are not\naccounted for in Archimedes’ method.\nReply: Thanks for your insightful comment.\nAs you commented, the presence of closed pores may affect the accuracy of the Archimedes\nmethod, which is the possible reason why some references reported high densities but also observed\nvisible pores. In our first response to you, we demonstrated a relatively dense surface and cross-section\nof our as-sintered samples, indicating that density was not the primary factor determining the\ndifferences in the piezoelectric properties of this work. Finally, we have added certain comments on\nthe effect of closed pores on the accuracy of the Archimedes method, as shown on Page 23 of the\nrevised Manuscript.\nComment 3. Neutron Diffraction and PDF Analysis:\nThank you for addressing my concerns and improving the analysis in this section. The revisions have\nclarified the interpretation.\nReply: Thank you very much for your insightful comment, which has greatly helped us improve our\nwork.\nComment 4. On “Convergent” and “Divergent” Displacements:\nThank you for the added explanation. The revised text is clearer in conveying the intended meaning.\nReply: Thank you very much for your insightful comment, which has greatly helped us improve our\nwork.\nComment 5. On Crystallography in M and O Phases:\nThank you for your response. The clarification on polarization directions and references provided are\nappropriate, and I find the explanation satisfactory.\nReply: Thank you very much for your insightful comment, which has greatly helped us improve our\nwork.\n6. DFT Calculations:\nThank you for the revisions and improvements made to this section. The clarification on atomic\nconfigurations has enhanced the credibility of the computational analysis.\nReply: Thank you very much for your insightful comment, which has greatly helped us improve our\nwork.\nR-7\nResponse to Reviewer #3:\nThe manuscript has been carefully revised according to the reviewers’ comments. I find the revisions\nsatisfactory and recommend acceptance in its present form.\nReply: Thanks for your insightful suggestions as well as your kind recognition and recommendation\nof our work.\nR-8"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The manuscript can be published in the present form."}, "author_response": "Response to Reviewers\n(Manuscript no: NCOMMS-25-48604)\nResponse to Reviewer #1:\nThe manuscript can be published in the present form.\nReply: Thanks for your insightful comments as well as your kind recognition and recommendation of\nour work.\nR-1"}]

Decision letter