Review Reports - Stabilization of Mixed-Anion (O<sup>2−</sup>/S<sup>2−</sup>) Networks in ZnO-Substituted Silicate–Phosphate Oxysulfide Glasses: Linking Cation–Sulfide Bonding to Thermal and Dielectric Properties

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper melts the sulfur in a strong reducing atmosphere to convert it into sulfides (S^2-), mainly studying the silicate-phosphate oxy-sulfide glass in the SiO₂-P₂O₅-K₂O-MgO-SO₃ system to determine how the substitution of MgO with ZnO under reducing conditions affects its thermal and electrical properties. The results show that the glass with zinc oxide substitution has lower dielectric constant, higher energy activation, lower AC conductivity, and reduced migration rate of alkali ions in the network. The substitution of zinc oxide for magnesium oxide enhances the network of reduced-state sulfur oxide glass. The research in this paper has good industrial value and is worthy of publication recommendation, but there are still some deficiencies that need to be improved.
1. In the first line of the abstract, the chemical formula "SiO2-P2O5-K2O-MgO-SO3" is wrongly written. Please correct the chemical formula.
2. The abstract should first introduce the research background of this paper, but it does not. Please add it. In addition, the research results do not have quantifiable data, and they are all qualitative conclusions. Please modify. Also, the research significance of this paper should be supplemented.
3. In Chapter 2, "Experimental Procedure", for the relevant parameters of the materials, it is recommended to write them in a list format, which makes it clear at a glance.
4. The author should, based on the current research status, explain in the research methods and significance how this paper fills the gaps and makes what innovative contributions.
5. Why does the increase in ZnO content reduce configurational heat capacity, inhibit crystallization exotherm, and increase melting point? What is the reason?
6. The author explains why the glass with zinc oxide substitution has lower dielectric constant, higher energy activation, and lower AC conductivity.
7. In Section 3.5, the equations cited, such as Formulas 1-3, need to be annotated with references. Additionally, some figures are not clear enough. Please improve the quality of the data.
8. The conclusion of this paper is too long and appears very verbose. According to the test items, please condense and simplify it by concentrating on specific results and point by point, to improve readability.

Author Response

Reviewer 1 This paper melts the sulfur in a strong reducing atmosphere to convert it into sulfides (S2-), mainly studying the silicate-phosphate oxy-sulfide glass in the SiO2-P2O5-K2O-MgO-SO3 system to determine how the substitution of MgO with ZnO under reducing conditions affects its thermal and electrical properties. The results show that the glass with zinc oxide substitution has lower dielectric constant, higher energy activation, lower AC conductivity, and reduced migration rate of alkali ions in the network. The substitution of zinc oxide for magnesium oxide enhances the network of reduced-state sulfur oxide glass. The research in this paper has good industrial value and is worthy of publication recommendation, but there are still some deficiencies that need to be improved.
1. In the first line of the abstract, the chemical formula "SiO2-P2O5-K2O-MgO-SO3" is wrongly written. Please correct the chemical formula.

Answer: We thank the Reviewer for pointing this out. The chemical formula in the first line of the Abstract has been corrected to SiO2-P2O5-K2O-MgO-SO3-ZnO in the revised manuscript.

2. The abstract should first introduce the research background of this paper, but it does not. Please add it. In addition, the research results do not have quantifiable data, and they are all qualitative conclusions. Please modify. Also, the research significance of this paper should be supplemented.

Answer: We thank the Reviewer for this constructive comment. The Abstract has been substantially revised in the revised manuscript. A brief research background has been added at the beginning of the Abstract to highlight the relevance of silicate–phosphate oxysulfide glasses. In addition, qualitative statements have been replaced or complemented with quantitative results, including numerical changes in configurational heat capacity, crystallization temperatures, and activation energies derived from DSC and impedance spectroscopy. Finally, the research significance has been clarified by explicitly stating the implications of ZnO substitution for enhancing thermal stability and electrical insulating behavior in functional oxysulfide glass systems.

In Chapter 2, "Experimental Procedure", for the relevant parameters of the materials, it is recommended to write them in a list format, which makes it clearat a glance.

Answer: We thank the Reviewer for this helpful suggestion. In the revised manuscript, Section 2 (Experimental Procedure) has been reorganized into clearly defined subsections (Sections 2.1–2.6). The key material compositions are now explicitly presented in list form, and the most relevant experimental parameters (e.g., DSC conditions) are summarized using a concise list format where appropriate. This restructuring improves clarity and readability and allows the essential experimental details to be identified at a glance, in line with the Reviewer’s recommendation.

The author should, based on the current research status, explain in the research methods and significance how this paper fills the gaps andmakes what innovative contributions.

Answer: Electrochemical Impedance Spectroscopy (EIS) is a key method for evaluating the ionic conductivity of glass and glass-ceramic materials. In this study, we investigate the influence of MgO substitution with ZnO under reducing conditions on the electrical behavior of the glass system. Analysis using electrical equivalent circuits reveals changes in the dominant ionic conduction mechanisms. The dielectric properties of the glasses are examined over a broad frequency and temperature range. The observed dielectric response demonstrates the temperature-dependent evolution of capacitance. Furthermore, the dielectric loss (tan δ) indicates enhanced localized polarization and short-range hopping processes within structurally heterogeneous Zn–O/S-rich regions, while long-range ion diffusion remains limited. 

Why does the increase inZnO content reduce configurational heat capacity, inhibit crystallization exotherm, and increase melting point? What is the reason?

Answer: We thank the reviewer for this insightful question. The observed reduction in configurational heat capacity (ΔCp), suppression and broadening of crystallization exotherms, and the upward shift of melting-related endothermic effects with increasing ZnO content are attributed to the intermediate role of Zn2+ in the glass network. Zinc forms relatively strong and directional Zn–O bonds and participates in intermediate or mixed network-forming structural units, which locally reinforce the glass structure and reduce the number of accessible configurational states near the glass transition. This increased network rigidity limits long-range atomic diffusion and slows crystallization kinetics, resulting in weaker and broadened crystallization exotherms shifted to higher temperatures. At elevated temperatures, the enhanced cohesive energy associated with Zn-containing structural units requires additional thermal energy to disrupt the network, leading to higher melting or decomposition temperatures. These interpretations are consistent with established literature on Zn-modified silicate and phosphate glass systems and with the DSC trends observed in the present study.

The author explains whythe glass with zinc oxide substitution has lower dielectric constant, higher energy activation, and lower AC conductivity.

Answer: The dielectric constant is strongly correlated with the AC electrical conductivity, as both properties are governed by charge carrier dynamics within the material. Zinc substitution significantly influences ion mobility by modifying the local structural environment and altering the short-range order. These structural changes affect the potential barriers for charge transport, leading to variations in the activation energy. The observed increase in activation energy can be attributed to microstructural factors, including an increased concentration of lattice defects and enhanced local disorder, which hinder long-range ion motion and modify polarization mechanisms.

In Section 3.5, the equations cited, such as Formulas 1-3, need to be annotated with references. Additionally, some figures are not clear enough. Please improve the quality of the data.

Answer: The appropriate references were added. The figures were improved for better visibility.

The conclusion of this paper is too long and appears very verbose.According to the test items, please condense and simplify it by concentrating on specific results and point by point, to improve readability.

Answer: The conclusion was corrected according to Reviewer’s suggestion.

Reviewer 2 Report

Comments and Suggestions for Authors

The reviewed work is of interest to researchers and consumers of complex multifunctional glass. The authors presented the technology for producing and studying silicate-phosphate oxysulfide glass in the SiO2-P2O5-K2O-MgO-SO3 system.

They also presented experimental data from XRD, XAFS, DSC, and EIS, demonstrating how partial substitution of MgO with ZnO affects its thermal and electrical properties.The results demonstrate the impact of changes in the complex glass composition on its thermal stability and dielectric characteristics, improving them.

However, the submitted article is not without its shortcomings, and in order to be published in the journal "Materials," the authors must respond to the reviewer's comments in the text of the article and make the necessary additions and changes:

1) In the Abstract section, replace "replacement of MgO with ZnO" with "partial substitution of MgO with ZnO" in lines 13 and 26.

2) I recommend including "partial substitution of MgO with ZnO" in the Keyword list.

3) In the Introduction, note the areas of practical application of silicate-phosphate oxysulfide glass, and formulate the purpose of the work at the end of the Introduction.

4) Expand the abbreviation DSC (line 130, Section 2.1).

5) Provide diffraction patterns XRD confirming the amorphous nature of all the obtained glasses (line 150, Section 3.1.)

6) In Section 3.3, provide the chemical formulas of all crystalline phases identified by the XRD method, as well as references to the International XRD Database with the card numbers of the corresponding phases.

7) Along with the diffraction patterns of zinc-free glasses in Fig. 2, provide the diffraction patterns of zinc-containing glasses obtained under the same conditions as the zinc-free glasses to confirm the amorphous state and the absence of crystalline phases in the zinc-containing glasses.

8) Match the title of section 3.4.1 with the caption to Fig. 3:

XAFS (X-ray Absorption Fine Structure) at the K-edge of potassium devitrificate glass 37-5S (with an explanation of the abbreviation).

9) In Fig. 3b provide reference XAFS spectra of K₂O and K₂S from literary sources for comparison and more accurate interpretation of O-K-S bonds.

10) In Fig. 4b provide reference XAFS spectra of MgO and MgS from literature sources for comparison and a more accurate interpretation of O-Mg-S bonds.

11) Expand the abbreviation EIS, line 347, Section 3.

12) The statement about the formation of Zn-O-S rigid domains (lines 573-574) is unsubstantiated, since the article lacks X-ray absorption spectra of zinc (Zn), oxygen (O), and sulfur (S).

13) The statement in lines 597-601 of the Conclusion section is incorrect, since XAFS spectra do not characterize crystallization and long-range order, but provide qualitative information on the short-range order of the element under study in the samples in the presence of reliable reference spectra from standard certified compounds.

After the questions and comments posed in the text of the article have been answered and the revised version has been re-examined, the article may be published in the journal "Materials."

Author Response

1) In the Abstract section, replace "replacement of MgO with ZnO" with "partial substitution of MgO with ZnO" in lines 13 and 26.

Answer: We thank the Reviewer for this editorial suggestion. The wording in the Abstract has been corrected, and the phrase “replacement of MgO with ZnO” has been replaced by “partial substitution of MgO with ZnO” at the indicated locations.

2) I recommend including "partial substitution of MgO with ZnO" in the Keyword list.

Answer: Thank you for your attention. Suggested key words have been introduced.

3) In the Introduction, note the areas of practical application of silicate-phosphate oxysulfide glass, and formulate the purpose of the work at the end of the Introduction.

Answer: We thank the Reviewer for this suggestion. The Introduction has been revised to explicitly discuss the practical relevance and application areas of silicate–phosphate oxysulfide glasses, including their use as solid electrolytes, dielectric materials, sealing glasses, and precursors for functional glass–ceramics. In addition, the purpose of the work has been clearly formulated at the end of the Introduction, explicitly stating the objectives of investigating the effects of ZnO substitution on thermal behavior, sulfur retention, local structure, and electrical properties.

4) Expand the abbreviation DSC (line 130, Section 2.1).

Answer: The abbreviation DSC has been expanded to Differential Scanning Calorimetry at its first occurrence in Section 2.1.

5) Provide diffraction patterns XRD confirming the amorphous nature of all the obtained glasses (line 150, Section 3.1.)

Answer: X-ray diffraction (XRD) patterns of all the obtained glasses have been added to the revised manuscript (Figure X) to confirm their amorphous nature. All samples exhibit a broad diffuse halo and no sharp diffraction peaks, which is characteristic of glassy materials.

Answer: We thank the Reviewer for this helpful suggestion. In the revised manuscript, Section 3.3 has been updated to explicitly report the chemical formulas of all crystalline phases identified by XRD, together with the corresponding ICDD reference card numbers used for phase identification and Rietveld refinement

Answer: In accordance with the reviewer’s suggestion, the X-ray diffraction (XRD) patterns of the zinc-containing glasses have been added together with those of the zinc-free glasses in Fig. 2. All diffraction patterns were obtained under identical experimental conditions and show a broad diffuse halo with no sharp diffraction peaks, confirming the amorphous nature of both zinc-free and zinc-containing glasses.

8) Match the title of section 3.4.1 with the caption to Fig. 3:

Answer: The Figure 3 caption (now Figure 4) has been corrected. Thank you for pointing this out.

XAFS (X-ray Absorption Fine Structure) at the K-edge of potassium devitrificate glass 37-5S (with an explanation of the abbreviation).

Answer: Following the reviewer’s comment on the XAFS method, we noted that the description of the potassium K-edge experiment was not sufficiently explicit in the original manuscript. The Experimental section has therefore been revised to clearly describe both potassium and magnesium K-edge measurements. To avoid ambiguity, the term XAFS has been removed as a technique descriptor and replaced by X-ray Absorption Spectroscopy (XAS), while XAFS is retained only as part of the official beamline name at Elettra. The sample designation has also been clarified.

In addition, in response to comment 9, reference K K-edge XANES spectra of K2O and K2S from the literature have been included in Fig. 3b for comparison. These reference spectra provide a more accurate basis for the interpretation of potassium local environments and O–K–S bonding in the devitrified glass.

9) In Fig. 3b provide reference XAFS spectra of K2O and K2S from literary sources for comparison and more accurate interpretation of O-K-S bonds.

Answer: We thank the reviewer for this valuable suggestion. In the revised manuscript, reference potassium K-edge XAFS spectra of K2O and K2S, together with additional potassium-containing compounds, have been added to Figure 4 (in original version Fig. 3 before adding Figure with XRD patterns) for comparison. These reference spectra are included to qualitatively benchmark the spectral features and energy positions of the glass spectrum against representative oxide- and sulfide-based potassium environments.

At the same time, the discussion has been carefully revised to avoid explicit assignment of O–K–S bonding configurations. While the reference spectra improve the qualitative interpretation of the potassium local environment, a definitive identification or quantification of mixed oxide–sulfide bonding would require complementary S-edge and/or O-edge XAS measurements and quantitative spectral analysis, which are beyond the scope of the present study. Accordingly, the interpretation in the manuscript is now limited to qualitative comparison and assessment of local ordering around K⁺ ions.

10) In Fig. 4b provide reference XAFS spectra of MgO and MgS from literature sources for comparison and a more accurate interpretation of O-Mg-S bonds.

Answer: We thank the Reviewer for this helpful suggestion. In the revised manuscript we have added Mg K-edge reference spectra of MgO and MgS (and MgSiO3) from the literature to Figure 5 (in original version Fig.4) for direct comparison with the Mg K-edge spectrum of the 37–5S glass devitrificate. These references are provided to qualitatively benchmark the spectral features and energy positions of the glass against representative oxide- and sulfide-based magnesium environments.

At the same time, the Mg K-edge discussion has been revised to avoid explicit assignment of O–Mg–S bonding configurations. While the inclusion of MgO and MgS references improves qualitative interpretation, definitive identification or quantification of mixed oxide–sulfide coordination would require complementary S-edge and/or O-edge XAS and detailed EXAFS analysis, which are beyond the scope of the present study. Accordingly, the manuscript limits the interpretation to qualitative assessment of local ordering and coordination heterogeneity around Mg2+ ions.

11) Expand the abbreviation EIS, line 347, Section 3.

Answer: We expanded the abbreviation EIS.

12) The statement about the formation of Zn-O-S rigid domains (lines 573-574) is unsubstantiated, since the article lacks X-ray absorption spectra of zinc (Zn), oxygen (O), and sulfur (S).

Answer: We thank the Reviewer for this important observation and agree with the assessment. The original statement referring to the formation of rigid Zn–O–S domains was not sufficiently supported by the experimental data, as the manuscript does not include Zn-, O-, or S-edge X-ray absorption measurements. The corresponding sentence has therefore been removed, and the discussion has been revised to avoid implying specific Zn–O–S bonding motifs. In the revised manuscript, the interpretation is restricted to a more general and data-supported description of the role of Zn2+ as an intermediate oxide that locally increases network rigidity and suppresses long-range ionic transport. No explicit Zn–O–S domain formation is claimed in the revised version.

Answer: We agree with the reviewer that X-ray absorption spectroscopy does not characterize crystallization or long-range order, but instead provides qualitative information on the short-range local structure of the absorbing element through comparison with reference spectra. The statement in the Conclusions has therefore been corrected to accurately reflect the scope and limitations of the XAS/XANES technique.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript investigates how partial substitution of MgO with ZnO affects the thermal stability, crystallization behavior, local structure, and dielectric/electrical properties of reduced silicate–phosphate oxysulfide glasses in the SiO₂–P₂O₅–K₂O–MgO–SO₃ system.

The study provides several novel elements, among them being a systematic comparison of MgO vs. ZnO in a reduced mixed-anion (O²⁻/S²⁻) glass system, which is rarely explored in the literature, and the use of K-edge and Mg-edge XAS to monitor in-situ structural ordering during devitrification**, which is uncommon for these compositions.

The authors explicitly target a known gap: The lack of systematic experimental correlations between K–S, Mg–S, and Zn–S bonding environments and their influence on thermal stability, crystallization pathways, and dielectric behaviour in potassium-based silicate–phosphate oxysulfide glasses. This is a meaningful and timely gap, especially given the growing interest in mixed-anion glasses for solid-state electrolytes and dielectric applications.

Compared with existing literature, this study adds a clear mechanistic explanation of how Zn acts as an intermediate oxide that increases network rigidity and suppresses crystallization—something only partially addressed in earlier ZnO-containing oxide glass studies, a combined thermal–structural–electrical perspective, whereas most previous studies focus on only one of these aspects, and direct XAS evidence that Zn-containing glasses resist medium-range ordering during heating, unlike Mg-containing glasses.

The methodology is generally solid, but several improvements are quite necessary to improve the work:

Regarding the DSC analysis, multiple heating rates and Kissinger analysis could quantify activation energies for crystallization.
The equivalent circuit fits of the electrical measurements are described but not shown in detail. Authors should include fit quality indicators (χ², residuals) and a clearer justification for the chosen circuit.
On the XRD of Zn-containing glasses: Only Zn-free glasses were devitrified and analyzed. Devitrification of Zn-containing glasses would directly confirm whether Zn suppresses crystallization or merely shifts it.
A few references on Zn volatilization and redox chemistry could strengthen the discussion.
Table 1 is informative but shows large deviations between nominal and measured compositions. The authors should comment more explicitly on the implications of these deviations.

Author Response

The methodology is generally solid, but several improvements are quite necessary to improve the work:

Regarding the DSC analysis, multiple heating rates and Kissinger analysis could quantify activation energies for crystallization.

Answer: We thank the reviewer for this valuable suggestion. We agree that DSC measurements performed at multiple heating rates combined with Kissinger analysis could provide quantitative activation energies for crystallization. However, in the present study DSC experiments were carried out at a single heating rate, and the thermal analysis was therefore intended to provide qualitative and comparative insight into the crystallization behavior of Zn-free and Zn-containing glasses. A detailed kinetic analysis of crystallization is beyond the scope of the present work and will be the subject of future investigations.

The equivalent circuit fits of the electrical measurements are described but not shown in detail. Authors should include fit quality indicators (χ², residuals) and a clearer justification for the chosen circuit.

Answer : To fit the equivalent circuit models, the freely available EIS Analyzer software was used. The quality of the fitting was evaluated using the coefficient of determination (R²), which reflects the relative deviation between the experimental impedance data and the theoretical model response. In the analyzed systems, the fitting accuracy is influenced by electrode–electrolyte interface effects as well as non-Debye relaxation phenomena. As a result, the obtained R² values show deviations below 10%, which is considered acceptable for impedance spectroscopy measurements of such materials. Additionally, all fitted data were validated using the Kramers–Kronig (K–K) test, confirming the physical consistency, linearity, and stability of the measured impedance spectra. The selected equivalent circuit model was chosen as a compromise between fitting accuracy and physical simplicity. Importantly, the adopted circuit enables a reliable comparison between different samples, highlighting variations in their electrical response, which indicates differences in their microstructural and physicochemical nature.

We add the sentence

On the XRD of Zn-containing glasses: Only Zn-free glasses were devitrified and analyzed. Devitrification of Zn-containing glasses would directly confirm whether Zn suppresses crystallization or merely shifts it.

As indicated by the DSC curves, Zn-containing glasses exhibit weaker and broader crystallization effects shifted to higher temperatures compared to Zn-free compositions, suggesting slower crystallization kinetics and enhanced thermal stability. Consequently, devitrification of Zn-containing glasses was not achieved under the heat-treatment conditions applied in this work. This limitation has been clarified in the revised manuscript.

A few references on Zn volatilization and redox chemistry could strengthen the discussion.

Answer: A few references were introduced to the text.

Table 1 is informative but shows large deviations between nominal and measured compositions. The authors should comment more explicitly on the implications of these deviations.

Answer: We thank the Reviewer for this valuable comment. In the revised manuscript, Section 3.1 has been expanded to explicitly address the deviations between nominal and measured compositions observed in Table 1. In particular, the substantial loss of ZnO in the Zn-containing glasses is now discussed in detail and attributed to the well-known volatility of zinc under reducing conditions at high temperatures. Additional literature has been cited to support this interpretation, including thermodynamic and kinetic studies demonstrating carbothermic and gas-phase reduction of ZnO and subsequent zinc volatilization during high-temperature processing [15,21,22].

The implications of these deviations are now clearly discussed: while a significant fraction of zinc is lost during melting, the remaining zinc still affects melting chemistry and contributes to enhanced sulfur retention in the glass. At the same time, the lower-than-nominal SO₃ contents in Zn-free glasses are attributed to partial sulfur volatilization under reducing conditions. To avoid ambiguity, Table 1 has been clarified, and values reported as 0.000 are now explicitly defined as being below the detection limit of the XRF method.

These revisions strengthen the discussion of composition–processing relationships and provide a clearer, literature-supported interpretation of the compositional deviations observed experimentally.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors submitted a second version of article 4113819 with additions and corrections based on the reviewer's comments. This updated version significantly improved the scientific understanding of complex multifunctional glasses and increased reader interest in the results obtained on the effect of substituting MgO with ZnO on the thermal behavior, sulfur retention, local structure, and electrical properties of silicate-phosphate oxysulfide glass SiO2-P2O5-K2O-MgO-SO3.

I believe that the revised article is suitable for publication in the journal "Materials."

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have made substantial improvements to the manuscript in comparison to the previous version. The revised text exhibits greater clarity and rigor, resulting in a more coherent scientific narrative. The relationships between structure and properties are now demonstrated in a more convincing manner, and the experimental sections are presented with increased detail and transparency.

The updated version of the manuscript offers a much clearer explanation of the mechanistic links among Zn substitution, mixed-anion bonding (including Zn–S and Zn–O–S interactions), network rigidity, suppression of crystallization, and dielectric behavior. This enhanced clarity strengthens the overall scientific argument of the work.

The section detailing electrical properties has been entirely reconstructed. It now includes an Arrhenius analysis of both σ_dc and τ, temperature–frequency maps, and a discussion of space-charge polarization. Additionally, a comparison between Zn-free and Zn-containing glasses is provided, along with an interpretation of β-relaxation and ion mobility.

Overall, the manuscript is significantly improved and is now considered scientifically sound.