Effect of Zr Doping on BNT–5BT Lead-Free Ceramics: Substitutional and Excess Incorporation Analysis
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe present study appears to be interesting and well structured.
Minor points to be addressed by the authors:
1) In the curves of Fig. 5 the frequency of the applied field is not given. Please indicate the frequency in the revised version.
2) It would be helpful for the reader if plots of the hysteresis curves are shown, indicating also the resulting information (coercive field etc.)
3) Authors use the density of the examined materials. How its value was determined via mass and dimensions, or by employing any mixture rule?
4) In the formula for the calculation of g33, real part of dielectric permittivity in included. Which values of permittivity did the authors used, frequency of the field and temperature should be indicated.
Concluding the present work can be accepted for publication after minor revision.
Author Response
Answers to the reviewer
We would like to acknowledge the reviewer for the positive evaluation and valuable comments for improving our work, which have reinforced and clarified our manuscript. Please, find our answers to the comments.
Reviewer 1
Comment 1: In the curves of Fig. 5 the frequency of the applied field is not given. Please indicate the frequency in the revised version.
Response: Thank you for your observation. We have added the frequency of the applied field in the caption in the revised manuscript.
Comment: 2: It would be helpful for the reader if plots of the hysteresis curves are shown, indicating also the resulting information (coercive field etc.).
Response: We appreciate this suggestion. The polarization–electric field (P–E) hysteresis loops have been included in the revised version, and the key parameters such as coercive field (Ec) and remanent polarization (Pr) are now explicitly indicated and discussed in the text.
Comment 3: Authors use the density of the examined materials. How its value was determined via mass and dimensions, or by employing any mixture rule?
Response: Thank you for pointing this out. The density of the samples was determined using the Archimedes method in distilled water. This clarification has been included in the experimental section of the revised manuscript.
Comment 4: In the formula for the calculation of g33, real part of dielectric permittivity is included. Which values of permittivity did the authors use, frequency of the field and temperature should be indicated.
Response: We thank the reviewer for this observation. The values of dielectric permittivity used for the calculation of g33, as well as the corresponding frequency (1 kHz) and temperature (room temperature), have been included in the revised version of the manuscript.
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsManuscript number: micro-3752620
The title, “Effect of Zr Doping on BNT-5BT Lead-Free Ceramics: Substitutional and Excess Incorporation Analysis,”The authors performed a substitutional modification and an additional doping with Zr in the BNT-5BT ceramic system, and investigated their effects on piezoelectric and other properties. Below are some mandatory comments for your consideration.
- The authors are advised to enhance the introduction by aligning it more closely with the objectives and scope of the present study.
- The authors are requested to clarify that the modification level is only 2 mol% in both cases. Given this low concentration, it is possible that impurity phases are below the detection limit of lab-based XRD. Can the authors comment on this?
- Figure 2 needs correction. While the experimental data seem appropriate, the convoluted data should be clearly distinguished, preferably using a dotted line. The authors should review and update the figure accordingly.
- The authors should include a discussion in the manuscript about the addition of Zr in BNT-0.5BT, specifically addressing its likely position in the crystal structure. This should also be mentioned as a concluding point.
- The EDS data presented are not appropriate. The authors are advised to repeat the measurements and present the results more clearly. For reference, see the reported results in the following study, which also identifies impurity peaks: 1016/j.ceramint.2025.01.056; 10.1016/j.jallcom.2025.181266
- The authors are encouraged to provide permittivity versus temperature data for the poled samples and to perform P–E loop measurements in order to evaluate the changes in ferroelectric behavior.
Author Response
Answers to the reviewer
We would like to acknowledge the reviewer for the positive evaluation and valuable comments for improving our work, which have reinforced and clarified our manuscript. Please, find our answers to the comments.
Reviewer 2
Comment 1: The authors are advised to enhance the introduction by aligning it more closely with the objectives and scope of the present study.
Response: We thank the reviewer for this helpful suggestion. The Introduction has been revised to better align with the specific goals of this study. In particular, we now clearly state our motivation for comparing two Zr incorporation strategies (substitutional and excess addition) in the BNT–5BT system. We emphasize the importance of understanding how each approach impacts the structure–property relationships relevant to dielectric, ferroelectric, and piezoelectric behavior. The revised text also highlights the relevance of this comparison for the development of lead-free ceramics for energy harvesting applications.
Comment 2: The authors are requested to clarify that the modification level is only 2 mol% in both cases. Given this low concentration, it is possible that impurity phases are below the detection limit of lab-based XRD. Can the authors comment on this?
Response: We thank the reviewer for this pertinent observation. We confirm that the maximum Zr modification level evaluated in the structural characterization was 2 mol% for both substitutional and excess addition routes (with the exception of the 4 mol% substitutional sample, which was only included in piezoelectric testing, as noted in the manuscript). At such low doping levels, the presence of secondary phases—particularly those associated with Zr-rich compounds—may indeed fall below the detection threshold of conventional laboratory-based XRD, which typically has a sensitivity limit of ~2–3 wt%.
To address this, we complemented XRD analysis with Raman spectroscopy, which is more sensitive to local structural distortions and minor phase contributions. In addition, the microstructural evaluation by FE-SEM/EDS revealed slight Zr segregation in some excess-doped samples, suggesting the possible formation of Zr-rich regions at the grain boundaries. We have included this clarification in the revised manuscript and acknowledged the limitation regarding the detectability of trace phases in the experimental section.
Comment 3: Figure 2 needs correction. While the experimental data seem appropriate, the convoluted data should be clearly distinguished, preferably using a dotted line. The authors should review and update the figure accordingly.
Response: We thank the reviewer for this suggestion. In the revised version of Figure 2, we have clearly differentiated the experimental Raman spectra from the fitted (convoluted) data. Specifically, the fitted curves are now displayed using a dotted line style, as recommended, to enhance visual clarity. This adjustment allows for easier distinction between the raw data and the fitting result, improving the interpretability of the vibrational analysis. The figure caption has also been updated accordingly.
Comment 4: The authors should include a discussion in the manuscript about the addition of Zr in BNT-0.5BTº addressing its likely position in the crystal structure. This should also be mentioned as a concluding point.
Response: We appreciate this important observation. In the revised manuscript, we have added a discussion addressing the likely position of Zr⁴⁺ ions in the BNT–0.5BT perovskite structure. Given the ionic radius similarity between Zr⁴⁺ (0.72 Å) and Ti⁴⁺ (0.605 Å) in octahedral coordination, and supported by literature reports, it is most likely that Zr substitutes at the B-site (Ti⁴⁺/Nb⁵⁺ sites) of the ABO₃ lattice, particularly under substitutional doping conditions. In the case of excess addition, ZrO₂ may remain partially segregated at the grain boundaries or form minor secondary phases not easily detected by XRD. This distinction is now explicitly stated in the discussion and reiterated in the conclusions.
Comment 5: The EDS data presented are not appropriate. The authors are advised to repeat the measurements and present the results more clearly. For reference, see the reported results in the following study, which also identifies impurity peaks: 1016/j.ceramint.2025.01.056; 10.1016/j.jallcom.2025.181266
Response: We thank the reviewer for this important observation. In the revised version of the manuscript, we have improved the quality and clarity of the EDS elemental maps. The new images, acquired with optimized contrast and resolution, clearly show the spatial distribution of the main elements. Notably, the Zr signal appears strongly concentrated in a localized region, suggesting the formation of a secondary Zr-rich phase.
This observation is now highlighted in the figure caption and discussed in the results section. While the secondary phase could not be detected by XRD—likely due to its low volume fraction and fine scale—it is consistent with the segregation behavior reported in the references provided by the reviewer. The presence of this Zr-rich region reinforces the hypothesis that excess addition of Zr can lead to local phase separation at or near the solubility limit.
Comment 6: The authors are encouraged to provide permittivity versus temperature data for the poled samples and to perform P–E loop measurements in order to evaluate the changes in ferroelectric behavior.
Response: We thank the reviewer for this constructive suggestion. In the revised version of the manuscript, we have included P–E hysteresis loop measurements for representative samples, which allow a clearer assessment of the changes in ferroelectric behavior induced by Zr doping. The loops were recorded at room temperature using a Sawer-Tower modified circuit, and key parameters such as remanent polarization (Pr) and coercive field (Ec) are now discussed in relation to the doping strategy.
Regarding the permittivity versus temperature measurements on polarized samples, we repeated the experiments as suggested. The results showed a slight reduction in the permittivity values and a steeper shoulder around 130 °C, likely associated with a phase transformation. However, the overall changes were minor in magnitude and did not significantly alter the interpretation of the material's dielectric behavior. For this reason, we felt that including these additional curves would not substantially enrich the discussion and chose not to incorporate them in the final version of the manuscript.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe paper investigates the effects of zirconium (Zr) doping on the structural, microstructural, and functional properties of BNT-5BT lead-free ceramics, with a focused comparison between two incorporation strategies: substitutional incorporation and excess addition. Overall, the experimental design is systematic, the data is comprehensive. The manuscript requires specific revisions to enhance scholarly rigor and readability.
- The 4 mol% substitutionally doped sample (BNT-5BT-4Zrsub) was excluded from structural analyses (e.g., XRD and Raman spectroscopy) and only evaluated for piezoelectric properties (Table 1). This omission creates a data gap, as the performance deterioration (e.g., d33 drop to 90 pC/N) lacks structural correlation. For instance, potential causes like secondary phase formation or solubility limits were not investigated, weakening the assessment of overdoping effects.
- Both doping strategies showed evidence of minor secondary phases (e.g., Ba6Ti17O40 and Zr-rich regions in EDS maps), but these were not systematically quantified or linked to property variations. The XRD analysis (Figure 1) did not detect these phases due to resolution limits, but the study failed to employ complementary techniques (e.g., TEM) to confirm their impact.
- Raman data (Figure 2) was described qualitatively (e.g., "band broadening" or "increased asymmetry"), but no quantitative metrics (e.g., peak shifts in wavenumber or full-width half-maximum) were provided. This omission weakens claims about structural distortions, such as the rhombohedral-to-tetragonal transition. For example, the band at ~305 cm⁻¹ was noted to intensify with excess Zr but not fitted to confirm tetragonal character evolution.
- Key mechanisms, such as how Zr⁴⁺ substitution enhances domain wall mobility or piezoelectric response, were not explored at an atomic scale. The study attributes improvements to "local lattice distortions" but neglects to cite computational studies or address ionic radius effects (Zr⁴⁺ vs. Ti⁴⁺), resulting in an oversimplified narrative.
- For instance, the dielectric behavior (Figure 5) showed a decrease in Tmax with substitutional doping, but this was not robustly linked to structural data (e.g., XRD phase evolution in Figure 1). The causal chain (e.g., "structural distortions → dielectric changes") was inadequately established, relying on descriptive rather than analytical language.
- Claims like "substitutional doping favors MPB retention" are not fully supported, as the 4 mol% sample's structural data is missing. Performance deterioration at high Zr was attributed to "secondary phases" without direct evidence from techniques like XRD or SEM.
Author Response
Answers to the reviewer
We would like to acknowledge the reviewer for the positive evaluation and valuable comments for improving our work, which have reinforced and clarified our manuscript. Please, find our answers to the comments.
Review 3
Comment 1: The 4 mol% substitutionally doped sample (BNT-5BT-4Zrsub) was excluded from structural analyses (e.g., XRD and Raman spectroscopy) and only evaluated for piezoelectric properties (Table 1). This omission creates a data gap, as the performance deterioration (e.g., d33 drop to 90 pC/N) lacks structural correlation. For instance, potential causes like secondary phase formation or solubility limits were not investigated, weakening the assessment of overdoping effects.
Response: We thank the reviewer for this thoughtful observation. In fact, the 4 mol% Zr-substituted sample was analyzed by X-ray diffraction, Raman spectroscopy, and SEM, in addition to its piezoelectric and ferroelectric properties (d33, g33, Ec, and Pr, see Table 1). However, the structural characterization did not reveal significant changes compared to the other compositions, nor did it provide additional insights into the observed deterioration of functional properties.
In particular, no clear evidence of secondary phase formation, peak splitting, or increased disorder was identified in the XRD or Raman spectra. SEM images also showed similar microstructural features to those of the lower-doped samples. Despite the sharp decrease in d33 and Pr values, the lack of detectable structural anomalies suggests that overdoping effects at this level may involve subtle changes beyond the resolution of the techniques employed.
We could clarified this point in the revised manuscript and acknowledge that more sensitive characterization methods (e.g., TEM or synchrotron XRD) may be required to elucidate the structural origin of the performance degradation at high Zr substitution levels.
Comment 2: Both doping strategies showed evidence of minor secondary phases (e.g., Ba6Ti17O40 and Zr-rich regions in EDS maps), but these were not systematically quantified or linked to property variations. The XRD analysis (Figure 1) did not detect these phases due to resolution limits, but the study failed to employ complementary techniques (e.g., TEM) to confirm their impact.
Response: We thank the reviewer for this constructive comment. As stated, no additional peaks corresponding to secondary phases were detected in the XRD patterns of any of the studied compositions, nor were characteristic Raman features observed that would indicate the presence of secondary crystalline phases.
However, SEM images combined with EDS elemental mapping revealed the presence of well-defined Zr-rich regions in the samples with excess Zr addition. These regions are morphologically distinguishable and chemically distinct, suggesting partial segregation. Nevertheless, due to the lack of corresponding diffraction signals, we could not confirm the presence of a crystallographically defined secondary phase.
We acknowledge that complementary high-resolution techniques such as TEM would be necessary to determine whether these Zr-rich areas correspond to amorphous or nanocrystalline segregated phases. This limitation has now been explicitly addressed in the revised manuscript.
Comment 3: Raman data (Figure 2) was described qualitatively (e.g., "band broadening" or "increased asymmetry"), but no quantitative metrics (e.g., peak shifts in wavenumber or full-width half-maximum) were provided. This omission weakens claims about structural distortions, such as the rhombohedral-to-tetragonal transition. For example, the band at ~305 cm⁻¹ was noted to intensify with excess Zr but not fitted to confirm tetragonal character evolution.
Response: We appreciate the reviewer’s insightful observation. In response, we have updated Figure 2 to include the fitted (convoluted) Raman spectra for representative samples. This allows a more quantitative interpretation of the vibrational modes associated with structural changes.
In particular, we now provide the position and full width at half maximum (FWHM) of the main bands, including the mode near 305 cm⁻¹. This band, which becomes more prominent with increasing excess Zr, has been fitted and its evolution analyzed. Although its growth is consistent with a partial enhancement of tetragonal features—as also suggested in the literature—we agree that this alone is not conclusive evidence of a complete phase transition.
To reflect this, we have revised the corresponding paragraph in the discussion to avoid overstatement and to clarify that the observed changes indicate a trend toward local structural distortion rather than a full rhombohedral-to-tetragonal transition. Quantitative data extracted from the fits are now included in the revised manuscript to support this interpretation.
Comment 4: Key mechanisms, such as how Zr4+ substitution enhances domain wall mobility or piezoelectric response, were not explored at an atomic scale. The study attributes improvements to "local lattice distortions" but neglects to cite computational studies or address ionic radius effects (Zr+4 vs. Ti4+), resulting in an oversimplified narrative.
We thank the reviewer for this valuable observation. In the revised manuscript, we have expanded the discussion on the underlying mechanisms governing the functional behavior of BNT–5BT ceramics doped with Zr, based on the differences observed between substitutional and excess incorporation routes.
Specifically, we now address the ionic radius mismatch between Zr⁴⁺ (0.72Å) and Ti4+ (0.605 Å), which leads to local lattice distortions when Zr is incorporated at the B-site. In the substitutionally doped samples, these distortions are accompanied by a significant increase in remanent polarization (Pr) and moderate coercive fields (Ec), as shown in Table 1, indicating enhanced domain wall mobility and polarization switching. These results are interpreted as a consequence of lattice softening and increased polarizability due to homogeneous Zr4+ incorporation into the perovskite matrix.
In contrast, the excess Zr samples show a different trend: although d33 increases moderately, Pr decreases and Ec increases. This behavior is consistent with a relaxor-like response arising from structural disorder, Zr-rich chemical inhomogeneities, and defect-induced internal fields, which hinder long-range ferroelectric order but lower the energy barrier for switching.
Moreover, the deterioration in piezoelectric performance at 4 mol% substitutional Zr confirms the existence of an optimal doping limit, beyond which secondary phases or defect accumulation negatively affect domain activity.
These expanded discussions now appear in the revised version of the manuscript (see Discussion, pp. 10–11), offering a more detailed and mechanism-driven interpretation of the observed trends.
Comment 5: For instance, the dielectric behavior (Figure 5) showed a decrease in Tmax with substitutional doping, but this was not robustly linked to structural data (e.g., XRD phase evolution in Figure 1). The causal chain (e.g., "structural distortions → dielectric changes") was inadequately established, relying on descriptive rather than analytical language.
Response: We appreciate the reviewer’s observation. In the revised manuscript, we clarified that the progressive merging of the (110)/(111) XRD peaks in substitutionally doped samples indicates a reduction in rhombohedral distortion and increased structural symmetry. This evolution is consistent with the observed decrease in Tmax, as lower distortion weakens long-range ferroelectric order. The link between structural changes and dielectric behavior is now explicitly discussed in the revised version.
Comment 6: Claims like "substitutional doping favors MPB retention" are not fully supported, as the 4 mol% sample's structural data is missing. Performance deterioration at high Zr was attributed to "secondary phases" without direct evidence from techniques like XRD or SEM.
Response: We appreciate the reviewer’s comment and agree that establishing the structural basis for the performance deterioration at high Zr content is essential. In the revised version, we clarify that the 4 mol% substitutionally doped sample (BNT–5BT–4Zrsub) was indeed evaluated by XRD, Raman spectroscopy, and SEM. However, these analyses did not reveal any additional diffraction peaks, mode splitting, or microstructural anomalies that could unambiguously indicate secondary phase formation or structural transitions. The XRD patterns remained consistent with a tetragonal symmetry, and Raman spectra showed no emergence of new modes.
Despite the lack of detectable structural changes, the 4 mol% sample exhibited a marked decline in both d₃₃ and Pᵣ (Table 1), indicating degraded ferroelectric performance. This suggests that overdoping may introduce point defects, local strain, or compositional inhomogeneities below the detection limit of the employed techniques. We now explicitly state in the manuscript that no direct evidence of secondary phases was obtained and that the claim of MPB retention is limited to compositions up to 2 mol% Zr. These clarifications have been incorporated into the discussion and conclusion sections.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsThis study evaluates the effect of excess and substitutional zirconium (Zr) incorporation on the structural, microstructural, and functional properties of lead-free ceramics based on the 0.95(Bi0.5Na0.5)TiO₃–0.05BaTiO3 (BNT–5BT) system.
- The introduction could be enhanced for wider readership by introducing more NBT-based publications on energy storage, such as: Journal of Advanced Dielectrics 13, no. 1 (2023): 2242003; Microstructures 3 (2023): 2023023; Energy Storage Materials 38 (2021): 113-120.
- The left edge of Figure 1 is not clear and may need to be cropped a little more. Additionally, are there any refinement data to support the coexistence of the R and T phases and their corresponding percentages?
- In Figure 5, the authors directly identify the dielectric anomaly as TF-R. Is there any reference to support this identification?
- Figure 5 does not mention the data frequency. Are there any temperature dependence data at different frequencies (e.g., 1 kHz, 10 kHz, 100 kHz)? These data are very important for demonstrating relaxor behavior.
- On page 8, line 228, the authors state "The decrease in Ec also indicates reduced energy barriers for switching, consistent with a more relaxor-like response." However, the decrease in Ec only applies to the 0.5Zr-2Zr excess samples. When compared to undoped samples, Ec actually increases. Therefore, please modify this statement to avoid confusion.
Author Response
Answers to the reviewer
We would like to acknowledge the reviewer for the positive evaluation and valuable comments for improving our work, which have reinforced and clarified our manuscript. Please, find our answers to the comments.
Review 4
Comment 1: The introduction could be enhanced for wider readership by introducing more NBT-based publications on energy storage, such as: Journal of Advanced Dielectrics 13, no. 1 (2023): 2242003; Microstructures 3 (2023): 2023023; Energy Storage Materials 38 (2021): 113-120.
Response: We thank the reviewer for this valuable suggestion. In the revised manuscript, we have incorporated a paragraph in the Introduction highlighting recent progress in the energy storage capabilities of BNT-based materials. The recommended references were included to broaden the context and emphasize the multifunctional potential of these ceramics beyond actuation and sensing. This addition strengthens the motivation for studying structure–property relationships under different doping strategies.
Comment 2: The left edge of Figure 1 is not clear and may need to be cropped a little more. Additionally, are there any refinement data to support the coexistence of the R and T phases and their corresponding percentages?
Response: We appreciate the reviewer’s observation. In the revised version, Figure 1 was updated to improve visual clarity, and the left edge was cropped to remove extraneous margins. Regarding the phase coexistence, we acknowledge that quantitative phase analysis via Rietveld refinement was not performed in this study. However, the coexistence of rhombohedral (R) and tetragonal (T) features is qualitatively inferred from the characteristic broadening and asymmetry of the (110)/(111) pseudo-cubic reflections in the XRD patterns, consistent with MPB behavior previously reported for BNT–BT systems. This qualitative interpretation has been clarified in the revised text, and a statement acknowledging the limitation of not including refinement data has been added.
Comment 3: In Figure 5, the authors directly identify the dielectric anomaly as TF-R. Is there any reference to support this identification?
Response: We thank the reviewer for this observation. In the revised manuscript, we now clarify that the dielectric anomaly labeled as TF–R corresponds to the ferroelectric-to-relaxor transition, as commonly reported in BNT–BT-based ceramics. Additional references have been included to support this identification [e.g., Takenaka, T.; Kei-ichi Maruyama, K.M.; Koichiro Sakata, K.S. (Bi1/2Na1/2)TiO3-BaTiO3 System for Lead-Free Piezoelectric Ceramics. Jpn. J. Appl. Phys. 1991, 30, 2236, doi:10.1143/JJAP.30.2236.).
Comment 4: Figure 5 does not mention the data frequency. Are there any temperature dependence data at different frequencies (e.g., 1 kHz, 10 kHz, 100 kHz)? These data are very important for demonstrating relaxor behavior.
Response: We thank the reviewer for this important observation. In the revised version, we have added the frequency (1 kHz) used during the dielectric measurements to the caption of Figure 5 and clarified it in the experimental section. Although only one frequency was included in the original manuscript, additional measurements were carried out at 1 kHz and 10 MHz for selected compositions. These data show a shift of the dielectric maximum (Tmax) toward higher temperatures with increasing frequency, consistent with the typical relaxor behavior of BNT-based ceramics. This observation is now briefly discussed in the text.
Comment 5: On page 8, line 228, the authors state "The decrease in Ec also indicates reduced energy barriers for switching, consistent with a more relaxor-like response." However, the decrease in Ec only applies to the 0.5Zr-2Zr excess samples. When compared to undoped samples, Ec actually increases. Therefore, please modify this statement to avoid confusion.
Response: We thank the reviewer for pointing out this inconsistency. The sentence has been revised to clarify that the observed decrease in coercive field (Ec) refers to the trend within the excess Zr series (0.5–2 mol%) and not relative to the undoped sample. The revised version avoids generalization and improves accuracy.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsRevised version satisfactorily addresses the points mentioned in the first report.
Thus, the paper can be accepted in its present form.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors have successfully addressed all the comments. I now recommend accepting the manuscript
Reviewer 4 Report
Comments and Suggestions for AuthorsThe authors have addressed all my comments.