Synergistic Homovalent and Heterovalent Substitution Effects on Piezoelectric and Relaxor Behavior in Lead-Free BaTiO3 Ceramics

This study investigates lead-free BaTiO 3 (BT) perovskite ceramics, unraveling the synergistic effects arising from simultaneous homovalent (Zr) and heterovalent (Nb) substitution. Focusing on piezoelectric, ferroelectric, and relaxor behaviors, this research employs a comprehensive suite of analyses, including temperature-dependent dielectric measurements, polarization-electric field hysteresis loops, and bipolar strain measurements. Significantly, our study unveils that the simultaneous substitution of Zr and Nb in the BT lattice induces room-temperature relaxor behavior at relatively low concentrations (5% Zr and 3% Nb), yielding higher permittivity and larger maximum polarization compared to single element (Zr or Nb) substituted BT relaxors. Bipolar strain measurements showcase substantial large-signal d 33* values (~250 pm/V) across a broad temperature range (–50 °C to 30 °C) for BT ceramics with simultaneous 5% Zr and 2% Nb substitution. This research advances understanding of homovalent and heterovalent substitution in BT ceramics and opens avenues for tailoring properties to suit specific applications.

Implementing a chemical substitution strategy enhances BT dielectric performance tailored for specific applications, such as elevated material constants (permittivity, pyro-and piezoelectric coefficients), superior thermal stability across a wide temperature range, and the modulation of polarization-electric field (P-E) and dielectric constant-field (ε-E, i.e. tunability) hysteresis loops [3][4][5][6].This enhancement can be achieved through homovalent (with charge 4+) or heterovalent (with charge 3+ or 5+) cation substitutions at the Ti-site of BT [7][8][9][10].As the initial substituent content increases (e.g., Zr 4+ or Nb 5+ ), the temperatures of the phase transitions between the FE phases rise, converging with the decreasing T C at the so-called "tricritical point" [11].With higher substituent contents, a diffuse phase transition (DPT) occurs from a rhombohedral FE to a cubic paraelectric phase [12,13].Upon further substitution, relaxor behavior emerges, characterized by Vogel-Fulcherlike frequency dispersion of the temperature corresponding to the permittivity maximum, (T m ), and dissociating T C from structural phase transitions [14].While relaxors exhibit cubic symmetry on a macroscopic scale, the local-scale structure retains its polar nature to some extent [15].Both DPT and relaxors present a slim hysteresis loop with higher permittivity close to room temperature, which makes them interesting for energy storage applications [12,16].
The manifestation of DPT or relaxor behavior, along with the corresponding polar order in BT perovskite, is intricately linked to the type of substitution applied [5].Generally, the larger ionic radius J o u r n a l P r e -p r o o f of homovalent substituents induces random strain fields that hinder B-site displacements, disrupting the long-range FE order and thus slimming down the P-E loop [10,17].Conversely, heterovalent substitutions introduce complexity by triggering various charge compensation mechanisms to maintain lattice electroneutrality [18,19].In such cases, the disruption of B-site correlation (and the related effects on the P-E loop) occurs through a FE domain pinning mechanism.This pinning mechanism is due to random-field defects created by polar defects in the lattice, which induce robust lattice disorder and lead to relaxor behavior [20,21].In our previous study [7], we delved into the origins of relaxor behavior in BT ceramics through homovalent (BaZr x Ti 1−x O 3 -BZrT) and heterovalent (BaNb x Ti 1−x O 3 -BNbT) solid solutions.The disruption of the long-range spatial correlation of Tication displacements, a pivotal mechanism driving the emergence of relaxor behavior in BT-based perovskites, revealed distinct origins for homovalent and heterovalent substituents.Remarkably, heterovalent substitution (Nb 5+ ) proved more effective in inducing relaxor behavior with a minimal substitution concentration of ~7%, compared to homovalent substitution (35% of Zr 4+ ) in BT.In fact, the presence of B-site vacancies (V Ti ′′′′ , in Kroger-Vink notation), and thus the presence of V Ti ′′′′ -4Nb 5+ defect clusters in BNbT ceramics, proven in our previous study, is expected to create strong polar random fields (RFs), influencing the direction of Ti 4+ and Nb 5+ cation displacements in the neighboring unit cells.This effect induces robust lattice disorder, leading to relaxor behavior even with a minimal amount of heterovalent substituent concentration.In contrast, in BZrT, only strain effects arise from differences in ionic radii, giving rise to non-polar regions, with the extent of this effect significantly smaller than the observed strain and electrostatic potential difference in BNbT.
Therefore, a substantially higher concentration of homovalent substituents is required compared to heterovalent substitution to induce relaxor behavior.In previous works, only B-site substitutions with one of those elements were investigated; to the best of the authors' knowledge, a systematic investigation of simultaneous Zr/Nb substitution in BT ceramics is still missing.The simultaneous Zr/Nb substitution in the BT lattice presents an intriguing avenue for tuning macroscopic dielectric, FE, and piezoelectric properties.It can be hypothesized that with a small amount of simultaneous Zr/Nb substitution, the strong RFs from V Ti ′′′′ -4Nb 5+ defects (polar nature) may interact with external electric fields on a larger length scale than non-polar strain-based RFs resulting from Zr 4+ , making the J o u r n a l P r e -p r o o f combination of both cations even more effective in disrupting Ti-O long-range correlation.
Consequently, relaxor behavior could be induced at lower substituent contents than BNbT, allowing simultaneous avoidance of solubility issues and reduction of leakage current by shielding charged defects with (Zr-centered) non-polar regions.
This study delves into the intricate effects of concurrent Zr and Nb substitution in BT perovskite ceramics, with a specific emphasis on piezoelectric, FE, and relaxor behaviors.Employing a systematic methodology involving three distinct substitution series, we explored the influence of different Zr and Nb ratios on the structure, microstructure, and electrical properties.
Following the initial mixing and wet processing, the powders underwent calcination at 1250°C for 5 hours, and subsequently they were granulated by adding 5 %weight of polyethylene glycol, pressed, and sintered at temperatures from 1400°C to 1450°C for 5 hours in oxygen atmosphere, depending on the specific composition being targeted.To replenish oxygen vacancies, a subsequent reoxidation annealing process was carried out at 1000 °C for 48 hours in oxygen atmosphere.

Sample characterization
J o u r n a l P r e -p r o o f Powder XRD analyses were conducted using the D2 Phaser instrument (Bruker, Germany) equipped with a Co-K α source, employing increments of 0.02° per step within the 20° to 80° 2 range.Raman measurements were performed using a WITec alpha300R spectrometer (WITec GmbH, Ulm, Germany) with an 1800 gr/mm grating and an EC Epiplan-Neofluar DIC objective (Zeiss, Germany), utilizing a 532 nm laser excitation with an intensity of 10 mW.Scanning Electron Microscope (SEM) images, coupled with energy-dispersive X-ray spectroscopy (EDX), were acquired using the Crossbeam 340 instrument (Zeiss, Germany).Temperature-dependent dielectric properties were investigated employing an Agilent 4184A precision LCR meter over a temperature range of -100 °C to 200 °C and at various frequencies spanning 10 Hz to 10 MHz.P-E and strain-electric field (S-E) loops were measured with a frequency of 10 Hz using an aixACCT TF 2000E tester with laser interferometer, covering temperatures ranging from -100 °C to 200 °C.

Results and Discussion
Figure 1 presents the XRD patterns of BaZr x Nb y Ti (1−x−5y/4) O 3 (BZNT x-y, here x and y are corresponding substitutions amount -in %mole -of Zr and Nb, respectively) ceramic samples, highlighting three distinct series with varying Zr/Nb ratio substitutions, as follows: Series-1 maintains a constant Nb content (y) at 2.5%, while systematically increasing the Zr content (x) to 10%, 20%, 30%, and 40%.In Series-2, the Zr content (x) remains fixed at 10%, while the Nb content (y) undergoes incremental changes to 2.5%, 5%, 7%, and 10%.Series-3 holds the Zr content (x) at 5%, progressively increasing the Nb content (y) to 1%, 2%, 3%, and 5%.All series of sintered ceramic samples exhibited the typical perovskite structure, which is confirmed by the XRD patterns as shown in Figure 1.Furthermore, the (200) peaks in 2theta, ranging from 43° to 47°, exhibited a gradual shift towards lower angles, signifying an expansion in lattice constant as Zr and Nb substitutions progressed in Series-1 and 2, respectively.The presence of (002) and (200) peaks in Figure 1b and d confirms the coexistence of tetragonal and cubic symmetry in the ceramics at room temperature (RT).Within Series-1, only the (002) peak is evident for ceramics containing over 10% Zr, indicating the absence of a tetragonal phase.In contrast, the merging of (002) T and (200) C peaks with increasing Nb content in J o u r n a l P r e -p r o o f Series-2 suggests a gradual shift from tetragonal to cubic symmetry.At RT, the BZNT ceramics may exhibit various structural phases, including rhombohedral, orthorhombic, tetragonal, and cubic, depending on the Zr/Nb content.Unlike other investigative techniques such as Raman spectroscopy [22], which is more suitable at discerning subtle changes in local crystalline order, the coherence length of XRD proves insufficient to resolve minute distortions even adopting a refinement process.
Nevertheless, lattice parameters and unit cell volumes were derived through XRD Rietveld refinement, as depicted in Figure S1 of the supporting information (SI).As anticipated, in both Series-1 and -2, the c lattice parameter (derived from the refinement of the tetragonal or cubic phase fraction) and unit cell volume exhibited a linear increase owing to the larger ionic radius of Zr 4+ (0.72 Å)/Nb 5+ (0.65 Å) in comparison to Ti 4+ (0.605 Å) cations.Additionally, the merging of ( 002) and ( 200) peaks, along with the corresponding phase fraction values for tetragonal and cubic symmetry were identified, which is J o u r n a l P r e -p r o o f presented in Table S1 of the SI.The initial BZNT 10-2.5 ceramic displayed approximately 34.3% tetragonal and 65.7% cubic phases, while the XRD patterns of the subsequent samples in Series-1 could be accurately fitted with only cubic symmetry.In Series-2, a phase fraction of tetragonal and cubic symmetry persisted with increasing Nb content.However, the c/a ratio (tetragonality) of those ceramics is close to 1.0002, indicating a disordered pseudocubic phase.
In contrast, the XRD pattern of Series-3 samples revealed a subtle intensity of a secondary phase peak, denoted by an asterisk symbol in Figure 1e, corresponding to residual BaCO 3 , as identified by JCPDS no.45-1471.The presence of BaCO 3 in those ceramics may be attributed to an incomplete calcination process or accuracy limitations in weighing (A/B ratio > 1). Figure 1f illustrates the evolution of (002) T , (022) O , and (200) C peaks with increasing Nb content.These peaks exhibited a complex behavior, suggesting the presence of a mixture of two or three phases in Series-3 ceramics.The lattice constant of Series-3 ceramics obtained from XRD Rietveld refinement are presented in Figure S1c.
The calculated c lattice parameter, derived from the tetragonal or cubic phase fraction, exhibited a linear increase with increasing Nb content, consistent with the trends observed in Series-2.Although the coherence length of XRD is insufficient to resolve small distortions (such as the local pseudocubic phase, or tetragonal and orthorhombic phases) for slight changes in Nb content, a qualitative XRD phase analysis was performed using Rietveld refinement, and phase fraction values are detailed in Table S1.The initial BZNT with 5% Zr and 1% Nb ceramic showed ~95% tetragonal and ~5% cubic phases, while BZNT 5-2 or BZNT 5-3 ceramics exhibited a blend of tetragonal (~76% or 63%, respectively) and orthorhombic (14% or 27%, respectively) phase fractions.For the highly Nbsubstituted BZNT 5-5 ceramics XRD pattern, Rietveld refinement was unable to be performed with two phases; it necessitated a combination of tetragonal, orthorhombic, and cubic phases.It is noteworthy that, based on our Raman analysis (discussed in Figure 7 below), no rhombohedral symmetry was observed.Therefore, rhombohedral phase fraction contribution was excluded in the Rietveld XRD refinement.
J o u r n a l P r e -p r o o f respectively.Similarly, Series-2 samples also display a relatively modest increase in grain size, progressing from 1.50 µm to 1.60 µm from 2.5% Nb to 10% Nb content, respectively.In stark contrast, Series-3 samples experience a significant reduction in grain size, plummeting from 3.25 µm to 1.10 µm with an increase in Nb substitution from 2.5% to 5.0%.This observation underscores the inhibitory effect of Nb substitution on grain growth in BZNT samples, which apparently is active only at low Zr contents.This effect is attributed to the diffusion of Nb into BT grain cores, resulting in the formation of Ti-rich grain boundaries and the subsequent suppression of grain growth [23].The presence of (111) twins is noticeable in the SEM micrograph of BZNT 10-2.5 and BZNT 5-2 ceramics, as shown in Figure 2a and 2d, respectively.These (111) twins are observed only in some grains, while they are rarely observed in BZNT 5-1 matrix grains.Similar twins are frequently found in BT ceramics and are attributed to the presence of a Ti-rich liquid phase above the eutectic  shifted down to well below -100 °C for higher Zr substitutions (beyond 20%), which lies below the minimum temperature limit of our experimental setup, rendering ε m peaks beyond this point unrecordable.Similarly, in Series-2, the ε m peak also exhibited a pronounced shift well below -100 °C for samples with higher Nb substitutions (above 2.5%), as depicted in Figure 3b.Notably, the threshold substitution concentration for relaxor behavior varies significantly between singlesubstituted (BZrT or BNbT) systems and their double-substituted counterpart (BZNT).In singlesubstituted systems, the threshold concentration is 7% for heterovalent (BNbT) and 35% Zr in homovalent (BZrT) systems [7].The relaxor behavior in the double substituted BZNT system manifests itself even with Zr and Nb substitution concentrations of 10% and 2.5%, respectively, demonstrating the effectiveness of co-substitution for achieving relaxor behavior.

J o u r n a l P r e -p r o o f
To elucidate the transition from conventional FE to relaxors in Series-3 samples, the diffusivity of dielectric properties was explored, as depicted in Figure 4a-d.In general, according to the Curie-Weiss law, conventional FE adhere to T C < T, while relaxor behavior deviates from this ideal [28].For a deeper understanding of this phenomenon, the reciprocal of the dielectric constant (1/ε r ) as a function of temperature at 10 kHz was examined for Series-3 ceramics.The Burns temperature (T B , the temperature at which polar regions on a nanometer scale, characterized by randomly distributed directions of dipole moments, begin to emerge as the sample cools down [29]), was found to gradually increase from 85 °C to 105 °C with increasing Nb substitution in Series-3, expect for the BZNT 5-5 ceramic.For temperatures higher than T B , the reciprocal permittivity follows the Curie-Weiss law with a linear dependency, which is best described as a paraelectric state of the material [30].
To delve further into the diffuseness of the phase transition within Series-3, a modified Curie-Weiss law was employed to estimate the degree of diffuseness, as follows: In accordance with this model, a γ value close to '1' signifies a normal FE material, while '2' aligns with the ideal relaxor [30,31].As such, the BZNT 5-3 and BZNT 5-5 samples exhibit larger γ values compared to BZNT 5-1 and BZNT 5-2, consistent with an enhanced relaxor behavior.
In Figure 3c, the γ value does not consistently increase with increased Nb substitution concentration, as evidenced by the lower γ value for BZNT 5-5 compared to BZNT 5-3.The complex behavior observed in BZNT 5-3, due to its broader ε m peak and the mixture of two FE crystal symmetries, may account for its higher γ value.This suggests that factors other than just Nb concentration, such as the nature of the polymorphic phases present, play a significant role in determining the γ value.
To further investigate the electrical characteristics, temperature-dependent P-E hysteresis loops were measured for the ceramic samples in Series-1 and Series-2 at a frequency of 10 Hz, as presented in Figure 5. Notably, the initial ceramic samples of both series, represented by BZNT 10-2.5, displayed rather slim hysteresis loops without a well-saturated polarization at both -100 °C and RT, as depicted in Figure 5a-b, respectively.This characteristic slim hysteresis behavior aligns with the typical attributes of a relaxor FE, consistent with our frequency-dependent ε r (T) measurements.The temperature-dependent maximum polarization (P max ) for Series-1 is illustrated in Figure 5c.As anticipated, the P max values exhibited a trend corresponding to the ε r measurements in Figure 3a.With an increase in Zr substitution from 10% to 40% in Series-1, the P max value dramatically decreased from 5.6 µC/cm² to 1.1 µC/cm² at RT, indicative of an increase in disorder within the samples, leading to fewer polarizable regions.In fact, the samples with the highest substituent content (BZNT 30-2.5 and BZNT 40-2.5)displayed paraelectric behavior at RT, consistent with the XRD analysis.The temperature-dependent P max values of Series-2 closely mirrored the behavior observed in Series-1.
With an increase in Nb substitution from 2.5% to 10, the P max values diminished from 5.6 µC/cm² to 1.0 µC/cm² at RT.The remnant polarization (P r ) profiles as functions of temperature for Series-1 and

J o u r n a l P r e -p r o o f
Series-2 are illustrated in Figure S3 of the SI.In the case of BZNT 10-2.5 ceramic, a slight increase in  J o u r n a l P r e -p r o o f substantial d 33 * values (~250 pm/V) across a broad temperature range (-50 °C to 30 °C).This phenomenon could be attributed to the presence of polymorphic phases in BZNT 5-2, characterized by a narrow temperature range and the associated proximity to FE-relaxor crossover [4,32], a characteristic also supported by the XRD analysis.
Raman spectroscopy stands as an invaluable technique for probing the influence of chemical substitution on the local structure, a factor that can ultimately impact the observed temperaturedependent dielectric response.In Figure 7, we present the Raman spectra of all series, recorded at RT, with selected Raman modes (from 1 to 6) highlighted as they pertain to lattice disorder in the BZNT ceramics.Within the Raman spectra of Series-1 and 2, as indicated in Figure 7a and 7b, no long-range correlation of Ti 4+ displacements was observed.Typically, in Raman spectroscopy, the presence of long-range correlation of Ti 4+ displacements would manifest itself as a sharp peak at 311 cm −1 (mode 1) and a dip at 180 cm −1 (mode 2) [17,20].The absence of these modes is indicative of the absence of long-range FE polar order in the system [22,33].Notably, mode 5 (at ~750 cm -1 ) is associated with ionic radii mismatches attributed to B-site substituents [17].As the concentration of Zr 4+ substitution increases in Series-1 ceramics, the intensity of mode 5 also increases and approaches the height of mode 4, signifying random replacement of Ti 4+ with Zr 4+ in the BT system (cf.Figure 7a).Note that mode 4 adheres to first-order Raman scattering rules and is classified as a symmetric octahedral breathing mode in BT ceramics [34].For Series-2 Raman spectra in Figure 7b, mode 6 is linked to a localized BO 6 oxygen breathing vibration in the vicinity of Ti vacancies and is thus associated with the presence of V Ti ′′′′ -4Nb 5+ defect complexes, which emerge as a charge compensation mechanism in BZNT ceramics [7,20].With an increase in Nb 5+ substitution concentration, the intensity of mode 6 gradually rises, indicating an increase in V Ti ′′′′ concentration within the sublattice.Raman spectra in Figures 7a and 7b clearly reveal a high degree of disorder in the Series-1 and 2 ceramics, even in the case of the lowest-substituted sample, namely BZNT 10-2.5.This observation is in accord with dielectric measurements and XRD analyses.
On the other hand, the Raman spectra of Series-3 samples (presented in Figure 7c), specifically BZNT 5-1 and BZNT 5-2 ceramics, exhibit a distinct rather sharp peak at ~311 cm −1 (mode 1) and a small dip J o u r n a l P r e -p r o o f at ~180 cm −1 (mode 2).As mentioned, these characteristics signify the presence of long-range correlation of Ti 4+ displacements, reminiscent of the typical FE behavior observed in BT ceramics [22].With an increase in Nb 5+ substitution beyond 3%, mode 6 peak becomes evident as well as the absence of modes 1 and 2, indicating the presence of V Ti ′′′′ within the sublattice.This observation aligns seamlessly with the frequency dispersion behavior illustrated in Figure 3c, where samples with more than 3% of Nb 5+ displayed relaxor behavior with short-range polar ordering.Temperaturedependent Raman measurements were conducted for BZNT 5-1, BZNT 5-2, and BZNT 10-2.5, as depicted in Figure S4 of the SI.The BZNT 5-1 and BZNT 5-2 ceramics exhibit FE Raman modes (sharp peak at ~311 cm −1 and a small dip at ~180 cm −1 ) until 100 °C for 1% Nb and 60°C for 2% Nb.
This indicates the presence of FE-paraelectric phase transition in BZNT 5-1 and BZNT 5-2 samples at 100 °C and 60°C, respectively, as illustrated in Figure S4a and b in the SI, and is in accord with the dielectric measurements in Figure 3c, which show the permittivity peak below those temperatures.In contrast, the BZNT 10-2.5 ceramic shows no indications of phase transitions, and typical FE features (modes 1 and 2) are absent at all temperatures, suggesting a persistent disruption of long-range FE order and high disorder in the ceramic, which is shown in Figure S4c of SI.
We have summarized the ε r , P max , and d 33 * values for all our samples within the compositional range of energies associated with the presence of heterogeneous interfaces within the ceramic bulk [4,35].
These energies may contribute to further flattening the energy landscape, in addition to the long-range J o u r n a l P r e -p r o o f ceramics, Zr substitution introduces non-polar strain-based RF regions (depicted in yellow shadings in Figure 9a), which have a weak and highly localized effect around the Zr atoms.Consequently, a large amount of Zr substitution is required to induce relaxor behavior.This extensive Zr substitution in BT ceramic fosters a fragmented short-range polar state below T B without complex defects in the lattice (see Figure 9a, bottom panels).The greater chemical stability of Zr ions reduces electron jumps between Ti 4+ and Ti 3+ , resulting in limited dielectric losses in the BZrT ceramics [36,37].This is exemplified in BZrT 20 ceramic, where the maximum ε r value at RT was observed in the ternary diagram in Figure 7a.On the other hand, heterovalent BNbT ceramics contain V Ti ′′′′ -4Nb 5+ defects, promoting the formation of robust polar RF regions (depicted in red shadings in Figure 9b).These regions interact with external electric fields on a larger length scale in the lattice, making them more effective in disrupting Ti-O long-range correlation even with a small amount of Nb% substitution content, as experimentally demonstrated in our previous studies [7,20].In contrast to BZrT, Nb substitution in BT ceramic promotes a highly disordered FE state with nanoscopic polar regions below T B as well as complex defects in the lattice (see Figure 9b, bottom panels), giving rise to high dielectric losses [7,38].
In the context of double substituted BZNT ceramics (refer to Figure 9c), a carefully chosen 5% Zr content promotes non-polar regions with weak strain-based RFs (yellow shadings), while Nb content generates robust polar RFs (red shadings) that interact with external electric fields on a larger length scale, influencing the long-range spatial correlation of Ti-cation displacements.Consequently, these non-polar regions resulting from Zr substitution can minimize the dielectric losses by shielding charged defects.Moreover, the relatively low levels of Zr and Nb content in mixed BZNT contribute to a reduction in the lattice defect complex, fostering relaxor behavior with a broad and high permittivity response resulting from multiple (Zr-and Nb-based) relaxation processes.This phenomenon is clearly manifested in BZNT 5-3 ceramic, where relaxor behavior is observed with a relatively large ε r and slim P-E loops, exhibiting the highest P max value compared to single-substituted BZrT 35 and BNbT 7 relaxors with minimal substitution concentration [7].This observation underscores the efficacy of employing a combination of homovalent (Zr) and heterovalent (Nb) substitutions to induce relaxor behavior in lead-free BT ceramics, achieving high permittivity and a J o u r n a l P r e -p r o o f instancefor energy storage applications.Furthermore, this combination of substituents (remarkably for Nb contents below 3%, for which FE phase is present) appears to promote high piezoelectric properties that are stable over a large temperature interval (-50 °C to 30 °C), which makes these materials attractive for underwater applications or in-body applications, especially considering the biocompatibility of BT-based compositions.

Conclusion
In summary, this comprehensive investigation into simultaneous homovalent (Zr) and heterovalent

Figure 2
Figure 2 presents SEM images showcasing the microstructures of selected samples within Series-1,

Figure 3
Figure 3 presents the relative permittivity as a function of temperature at various frequencies ranging

Figure 3 .
Figure 3. (a-c) Temperature-dependent relative permittivity at different frequencies of BZNT samples for Series-1, Series-2, and Series-3, respectively J o u r n a l P r e -p r o o f

Figure 5 .
Figure 5. Polarization-Electric field (P-E) loops of Series-1 samples measured at (a) -100 °C and (b) RT.(c) Temperature-dependent maximum polarization (P max ) values of Series-1 samples.P-E loops of Series-2 samples measured at (d) -100 °C and (e) RT.(f) Temperature-dependent P max values of Series-2 samples.J o u r n a l P r e -p r o o f

Figure 7 .
Figure 7. Room temperature Raman spectra of (a) Series-1, (b) Series-2, and (c) Series-3 of BZNT samples.The peak marked with an asterisk for the BZNT 10-5 composition denotes the secondary phase.

BaTiO 3 -
BaZrO 3 -BaNbO 3 , as illustrated in Figure8with color-coded data markers superimposed on the ternary phase diagram.Note that BaNbO 3 is a fictitious compound, and its use in the ternary diagram is just for the sake of consistency in naming.The accompanying bar next to the phase diagram correlates the color of the data marker with the magnitude of (a) ε r , (b) P max , and (c)d 33 * atRT.The highest ε r is observed in the region with 20% Zr and 0% Nb, attributed to the proximity of the temperature (T m ) at which the ε r (7500) maximum occurs to RT, typically forming either a tetragonal phase or a coexistence of tetragonal and orthorhombic phases[7].Another region of the diagram with high permittivity, however, is in correspondence of the compositions with 5% Zr and up to 2% Nb, which are associated also with superior piezoelectric properties.Notably, the BT ceramic with 5% Zr and 2% Nb (BZNT 5-2) exhibits the highest P max (11.87 µC/cm 2 ) and d 33 * (287 pm/V) properties.Furthermore, it is evident that a slight increase in Nb content beyond 2% along the 5% Zr line in the ternary diagram results in a decrease in ε r , P max , and d 33 * values.This underscores that even a relatively J o u r n a l P r e -p r o o f small concentration of Nb can induce significant changes in piezoelectric and dielectric properties.Along the 5% Zr line in the ternary diagram, a gradual increase in Nb up to 2% leads to a broadening of the transition T C or T m range, resulting in a DPT and corresponding composition-induced FE-relaxor crossover behavior at RT, which might be responsible for the highest P max value in BZNT 5-2 ceramic solid solution.Furthermore, the XRD and Raman analyses of BZNT 5-2 sample indicate the existence of long-range FE phase transitions and nanoscale structural heterogeneity.These distinctive features in BZNT 5-2 may contribute to additional energy contributions, including electrostatic and elastic

Figure 8 .Figure 9 .
Figure 8.The triangle ternary diagram of BaTiO 3 -BaNbO 3 -BaZrO 3 shows (a) Ɛr, (b) P max , (c) d 33 * at RT. J o u r n a l P r e -p r o o f

(
Nb) substitution in lead-free BT perovskite ceramics has yielded valuable insights into their structural, microstructural, and electrical characteristics.XRD analysis indicated the formation of a single-phase perovskite solid solution, revealing an increase in lattice parameters with increasing Zr or Nb content across all ceramic series.Series-3 samples displayed a mixture of perovskite phases with different crystal symmetries, as unveiled by XRD refinement.The temperature-dependent ε m behavior in Series-1 and 2 ceramics showed frequency dispersion mostly below RT, with the ε m peak shifting to lower temperatures as Zr or Nb content increased.Remarkably, in Series-3, specifically in BZNT samples with a constant 5% Zr and increasing Nb from 1% to 2%, the ε m peak extended at and above RT with DPT and no relaxor behavior.The BZNT 5-3 ceramic exhibited relaxor behavior at relatively low concentrations (5% Zr and 3% Nb), corroborated by frequency-dependent ε r measurements and modified Curie-Weiss law analyses.Furthermore, this relaxor composition exhibited higher permittivity and maximum polarization than single-substituted BZrT 35 or BNbT 7 relaxors, which makes it very interesting for dielectric applications.Notably, BT ceramics with 5% Zr and 2% Nb substitution displayed substantial d 33 * values (~250 pm/V) observed across a broad temperature range (-50°C to 30°C).This enhancement is likely due to the presence of polymorphic phases in BZNT 5-2, characterized by a narrow temperature range and proximity to the FE-relaxor crossover.This unique characteristic makes the BZNT 5-2 composition particularly promising for piezoelectric applications.The proposed schematic representation of non-polar and polar lattice structures induced by Zr and Nb substituents, respectively, emphasizes the effectiveness of simultaneous Zr/Nb substitution in disrupting long-range FE behavior and attaining interesting properties.This investigation underscores J o u r n a l P r e -p r o o f