Phase Transformations upon Formation of Transparent Lithium Alumosilicate Glass-Ceramics Nucleated by Yttrium Niobates

Round 1

Reviewer 1 Report

Very interesting study on the use of yttrium niobate as a nucleating agent for LAS glass-ceramics. The focus is therefore on the role of yttrium niobate in its polymorphic forms and the subsequent crystallization of the high quartz solid solution. I have the following comments:

1) The motivation for doping the glass or the high quartz solid solution with small amounts of cobalt is unclear.

2) Why was so much Y2O3 and Nb2O3, together 10 mol%, used in the batch? Both raw materials are expensive, aren't they? Their proportions are clearly above the proportions of TiO/ZrO2 usually used in industry.  

3) It looks like two different diffractometers were used. In the first section of chapter 2.2.2. it says Shimadzu XRD 6000 while in the third section it says Shimadzu XRD 7000.  In addition, for the room temperature measurements the measurement conditions (step size, scan rate, detector, etc.) are missing and for the HT measurement the type of X-ray tube (also copper?).

4) In chapter 2.2.6. it says "on the same samples ..." Which ones are meant here?

5) The caption to Figure 1 should state the thickness of the specimens. The text lacks a description of the results for Figure 1. Among other things, it is unclear whether the last sample is purple again and why?

6) The caption to Figure 2 is missing the heating rate specification.

7) The assignment of the peaks to the crystal phases in Figure 3a should be marked with symbols. This will save the reader from having to draw vertical lines through the diagram to find this out for themselves.

8) Figure 3, pie plot: quanified fractions in wt% ?, mol% ? or vol%?

9) Table 1: Uncertainty (error) of all parameters is missing.

10) In the caption to figure 4 the assignment to part (a) and part (b) is missing.

11) Lines 412-413: Assignment of the reference Bogdanova et al. is done with [29]. In the list of references, however, this article is listed as [40]. In addition, the article is in Russian and not accessible to the reviewer. Is there a translated version or an alternative article on this topic to cite?

Author Response

We are very grateful to Reviewer 2 for valuable comments.

Reviewer 1 comments:

Very interesting study on the use of yttrium niobate as a nucleating agent for LAS glass-ceramics. The focus is therefore on the role of yttrium niobate in its polymorphic forms and the subsequent crystallization of the high quartz solid solution. I have the following comments:

1) The motivation for doping the glass or the high quartz solid solution with small amounts of cobalt is unclear.

Thank you very much for this question.

The motivation for doping the glass and, consequently, after heat-treatments, the high quartz solid solution, with small amounts of cobalt is to demonstrate that the developed glass-ceramics are multifunctional materials. In this work, we have shown that cobalt ions, as representatives of transition metal ions, do not enter the crystalline phase of yttrium orthoniobate, but selectively enter crystals of the high quartz solid solutions. RE ions, such as Er³⁺, Yb³⁺, Ho³⁺, Tm³⁺, Eu³⁺, do not enter the structure of the high quartz solid solution, but form their own phase of RENbO₄ or selectively enter the structure of yttrium orthoniobates [6,7]. By this means, rare-earth niobates play the dual role of nucleating agents and active crystalline phases containing rare-earth ions. In this paper, we demonstrate a selective doping of rare-earth and transition metal ions into different crystalline phases.

The following paragraph is added to Discussion:

Doping the glass with small amount of cobalt oxide allowed us to demonstrate that the developed glass-ceramics are multifunctional materials. In this work, we have shown that cobalt ions, as representatives of transition metal ions, do not enter the crystalline phase of yttrium orthoniobate, but selectively enter crystals of the high quartz solid solutions. According to our previous studies [6,7] rare-earth ions, such as Er³⁺, Yb³⁺, and Eu³⁺, do not enter the structure of the β-quartz ss, but form their own phase of rare-earth orthoniobate or selectively enter the structure of yttrium orthoniobate [6,7]. By this means, rare-earth orthoniobates play the dual role of nucleating agents and active crystalline phases containing rare-earth ions. Thus we demonstrated the development of transparent glass-ceramics with near zero thermal expansion and with a selective doping of rare-earth and transition metal ions into different crystalline phases.

2) Why was so much Y₂O₃ and Nb₂O₃, together 10 mol%, used in the batch? Both raw materials are expensive, aren't they? Their proportions are clearly above the proportions of TiO/ZrO₂ usually used in industry.  

1. The glass composition studied in this work is the model composition. In this study, we aimed to reveal the mechanism of phase transformations in yttrium orthoniobates upon heating the initial glass and cooling the fabricated glass-ceramic. The relatively high content of this crystalline phase made it easier for us to study these phase transformations. For the development of particular luminescent materials we used lower contents rare-earth ions and niobium oxide, i.e., in ref. [6], the contents of Yb₂O₃ and Nb₂O₃ together are 6 mol%.
2. The phase of YNbO₄ is not only the nucleating agent but also an active crystalline phase containing RE ions. That is why it is quite natural that the proportion of Y₂O₃ and Nb₂O₅ is above the proportions of TiO₂/ZrO₂ usually used in industry. It should be noted that RE ions cannot be doped to the main crystalline phase of LAS glass-ceramics used in industry.

The following paragraph was added to Discussion:

In this study, we aimed to reveal the mechanism of phase transformations in yttrium orthoniobate nanocrystals upon heating the initial glass and cooling the fabricated glass-ceramic. The glass composition studied in this work was the model one. Relatively high concentrations of yttrium and niobium oxides made it easier for us to study these phase transformations. It should be noted that the crystalline phase of YNbO₄ is not only the nucleating agent but also a promising host for other rare-earth ions [6,7]. That is why it is quite natural that the proportion of Y₂O₃ and Nb₂O₅ is above the proportions of TiO₂/ZrO₂ usually used in industry. It should be noted that rare-earth ions do not enter any crystalline phase of LAS glass-ceramics produced in industry.

3) It looks like two different diffractometers were used. In the first section of chapter 2.2.2. it says Shimadzu XRD 6000 while in the third section it says Shimadzu XRD 7000.  In addition, for the room temperature measurements the measurement conditions (step size, scan rate, detector, etc.) are missing and for the HT measurement the type of X-ray tube (also copper?).

Thank you very much for this inquiry. Indeed, for the room temperature and in-situ studies we used two different diffractometers.

We made the following correction of the text: High-Temperature powder X-ray diffraction (HT-PXRD) patterns were recorded with a Shimadzu XRD-7000 diffractometer with CuKα radiation (λ = 1.5406 Å) equipped with an Anton Paar HTK-1200 furnace attachment.

4) In chapter 2.2.6. it says "on the same samples ..." Which ones are meant here?

Thank you very much for this inquiry.

We made the following correction of the text: Absorption spectra were measured on a Shimadzu UV-3600 spectrophotometer in the spectral range from 200 to 3300 nm on the same plane-parallel polished samples with a thickness of ~1 mm that were used for the Raman spectra recording.

5) The caption to Figure 1 should state the thickness of the specimens. The text lacks a description of the results for Figure 1. Among other things, it is unclear whether the last sample is purple again and why?

Thank you very much for this inquiry. We made the following correction of the Figure 1 caption:

Figure 1. Images of samples of the initial and heat-treated glasses. The thickness of polished transparent samples is 1 mm. The values indicated in the figure denote heat-treatment schedules.

The last sample is opaque and has a slight violet color tint, which is the result of light absorption and scattering losses. The following correction is added to the text:

The images of polished samples of transparent initial glass and glass-ceramics with the thickness of 1mm and of the opaque sample prepared by the heat-treatment at 700 °C and at 1350 °C for 6 h at each stage are shown in Figure 1.

The following correction of section 3.5. Absorption, is made:

Absorption spectra of the initial glass and glass-ceramics are determined by absorption of Co²⁺ ions. The spectrum of the initial blue glass shows three wide absorption bands in the visible region at 510, 590 and 650 nm, and a weak broad band in IR region spanning from ~1000 to ~2300 nm with a maximum at ~1460 nm, see Figure 9. The shape of the absorption spectrum remains near unchanged after heat-treatments at 700 ^oC and 700 + 750 ^oC for 6 h, these samples are also blue-colored, see Figure 1. When the main crystalline phase of β-quartz ss starts to crystallize at 800 ^oC, the material becomes violet-colored, and its absorption drastically changes. It is characterized by a number of well-resolved intense bands in the visible range of spectrum with maxima at 505, 550 580 and 595 nm, see Figure 9. There is also a weak inflection at 430 nm. In IR region, broad bands are observed with maxima around 1250, 1500, 1720, and 1970 nm. The spectrum remains near unchanged up to heat-treatment at 700 + 1000 ^oC, when the color of the materials changes to violet-blue, and new spectral features evolve. In the visible region, the spectrum is characterized by a number of narrow bands at 545, 580 and 620 nm. In the IR, there are three wide bands at approximately 1260, ~1400, and 1560 nm.

 6) The caption to Figure 2 is missing the heating rate specification.

Thank you very much for this inquiry. We made the following correction of the Figure 2 caption:

Figure 2. The DTA curve of the initial glass recorded in heating (red curve) and cooling (blue curve) modes. The heating and cooling rates are 30 °C/min. The values indicated in the figure denote the temperature, in °C.

7) The assignment of the peaks to the crystal phases in Figure 3a should be marked with symbols. This will save the reader from having to draw vertical lines through the diagram to find this out for themselves.

Thank you very much for this inquiry. The correction is made.

8) Figure 3, pie plot: quanified fractions in wt% ?, mol% ? or vol%?

Thank you very much for this inquiry. The correction is made: The pie plot represents the quantified phase composition (mass%)…

9) Table 1: Uncertainty (error) of all parameters is missing.

The errors are listed in the section “Materials and Methods”, subsection 2.2.2. Powder X-ray diffraction (PXRD).

Nevertheless, the correction is made:

Table 1. Characteristics of crystalline phases of yttrium niobate and β-quartz ss in LAS glass-ceramics prepared by different heat-treatments.

10) In the caption to figure 4 the assignment to part (a) and part (b) is missing.

Thank you very much. The correction is made: Figure 4. In-situ PXRD patterns collected during (a) the heating of the initial glass and (b) the cooling of the fabricated glass-ceramic..

11) Lines 412-413: Assignment of the reference Bogdanova et al. is done with [29]. In the list of references, however, this article is listed as [40].

Thank you very much. The correction is made: Following Bogdanova et al. [40], we state [23] that Co²⁺ ions isomorphously replace Li⁺ ions located mostly in octahedral sites in the disordered structure of β-quartz ss crystallized in glass having high viscosity.

In addition, the article is in Russian and not accessible to the reviewer. Is there a translated version or an alternative article on this topic to cite?

We found mentioning of this article in J. Am. Ceram. Soc. 1971, 54(8), 193d, https://ceramics.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1151-2916.1971.tb12322.x, and corrected the reference [40] in the following way:

40. Bogdanova, G.C.; Antonova, C.L.; Dzhurinskii B.F. Distribution of coloring ions in the structure of glass-ceramics. Izv. Akad. Nauk SSSR Neorg. Mater. 1969, 5, 204–206 (in J. Am. Ceram. Soc. 1971, 54(8), 193d).

Thank you very much indeed for your careful reviewing which helped us to improve the quality of the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

The study and the idea are very interesting but it has been largely studied. The novelty of this work is still low.

1- The authors said that the study "of phase transformations in transparent glass-ceramics containing rare-earth and niobium oxides" haven't reported (Here, for the first time, at line 94 page 2 ). Are the authors have any arguments for this ?

see the refs; https://ceramics.onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1997.tb02905.x

https://www.sciencedirect.com/science/article/abs/pii/0025540877900848

https://www.sciencedirect.com/science/article/abs/pii/0956715195901728?via%3Dihub

https://link.springer.com/chapter/10.1007/978-1-4684-1689-3_35

https://www.sciencedirect.com/science/article/abs/pii/S0272884212012448?via%3Dihub

https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.13095

2- The authors must rewrite some sentences, for instance;

- lines 24-25 page 1 < but at cooling the glass-ceramics after such heat-treatments, when YNbO4 nanocrystals with tetragonal structure undergo the second-order transformation at ~550 °C. >

- lines 59-61 page 2 <However, it was not clear up to now, if the monoclinic rare-earth orthoniobates were formed during the high-temperature heat-treatments or upon cooling down the glass-ceramics prepared by these heat-treatments.>

- line 93-94 page 2 <... and cooled it is monoclinic. Here, for the first time...>

3- Discussion section is not deeply traited the obtained results and linked it to the possible applications.

4- This can be more interesting by studying the doping effect and investigating some others properties as ferroelasticity

See comments for authors

Author Response

We are very grateful to Reviewer 2 for valuable comments.

Reviewer 2 comments:

The study and the idea are very interesting but it has been largely studied. The novelty of this work is still low.

Thank you for your comment. However, we cannot agree with you because it is the first time that transparent lithium alumosilicate glass–ceramics with near-zero thermal expansion nucleated by yttrium niobates are developed and phase transformations upon their formation are studied. In these glass-ceramics yttrium niobate is not only a nucleating agent but also a promising host for rare-earth ions.

1- The authors said that the study "of phase transformations in transparent glass-ceramics containing rare-earth and niobium oxides" haven't reported (Here, for the first time, at line 94 page 2 ). Are the authors have any arguments for this ?

see the refs; https://ceramics.onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1997.tb02905.x

https://www.sciencedirect.com/science/article/abs/pii/0025540877900848

https://www.sciencedirect.com/science/article/abs/pii/0956715195901728?via%3Dihub

https://link.springer.com/chapter/10.1007/978-1-4684-1689-3_35

https://www.sciencedirect.com/science/article/abs/pii/S0272884212012448?via%3Dihub

https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.13095

Thank you very much for these references. Of course, we are familiar with them.

Your first ref. has the title “Monoclinic-to-Tetragonal Phase Transformation in a Ceramic Rare-Earth Orthoniobate, LaNbO4”;

Your second ref. with the title “FERROELASTICITY IN THE LnNbO4-TYPE RARE EARTH NIOBATES” describes the monoclinic to tetragonal phase transition.

Your third ref. has the title “Electron back scattering study of domain structure in monoclinic phase of a rare-earth orthoniobate LaNbO4”.

Your forth ref. has the title “Crystal Growth and Optical Characteristics of Lanthanian Fergusonite”.

Your fifth ref. has the title “High temperature monoclinic-to-tetragonal phase transition in magnesium doped lanthanum ortho-niobate”.

Your last reference has the title “High-Temperature Properties and Ferroelastic Phase Transitions in Rare-Earth Niobates (LnNbO4)” and studies … phase transition from a monoclinic (S.G. I2/a) phase at low temperatures to a tetragonal (S.G. I41/a) phase at high temperatures.

Therefore, all these references describe studies of monoclinic to tetragonal phase transitions upon heating of the rare-earth niobate ceramics or powders with fergusonite structure.

Our article is the study of glasses (!) containing nanocrystals of yttrium niobate with disordered tetragonal T’ (not monoclinic!) structure. Upon heating to 1000 °C and above, these nanocrystals grow in size and acquire an ordered tetragonal sheelite-like structure. The sheelite-like structure exhibits transformation to a monoclinic structure upon cooling the glass-ceramics below 600 °C. The composition of the initial glass and its heat-treatment schedules are our inventions. The phase transformations of yttrium niobate with disordered tetragonal structure in invented glass-ceramics are studied in situ for the first time. The structure of these materials is studied for the first time.

2- The authors must rewrite some sentences, for instance;

- lines 24-25 page 1 < but at cooling the glass-ceramics after such heat-treatments, when YNbO4 nanocrystals with tetragonal structure undergo the second-order transformation at ~550 °C. >

- lines 59-61 page 2 <However, it was not clear up to now, if the monoclinic rare-earth orthoniobates were formed during the high-temperature heat-treatments or upon cooling down the glass-ceramics prepared by these heat-treatments.>

- line 93-94 page 2 <... and cooled it is monoclinic. Here, for the first time...>

We hope we convinced you that we can leave all these phrases unchanged because they are correct.

3- Discussion section is not deeply traited the obtained results and linked it to the possible applications.

Thank you very much for these comments. The results are partly discussed in the Result section. The following paragraphs are added to the Discussion.

In this study, we aimed to reveal the mechanism of phase transformations in yttrium orthoniobate nanocrystals upon heating the initial glass and cooling the fabricated glass-ceramic. The glass composition studied in this work was the model one. Relatively high concentrations of yttrium and niobium oxides made it easier for us to study these phase transformations. It should be noted that the crystalline phase of YNbO₄ is not only the nucleating agent but also a promising host for other rare-earth ions [6,7]. That is why it is quite natural that the proportion of Y₂O₃ and Nb₂O₅ is above the proportions of TiO₂/ZrO₂ usually used in industry. It should be noted that rare-earth ions do not enter any crystalline phase of LAS glass-ceramics produced in industry.

Doping the glass with small amount of cobalt oxide allowed us to demonstrate that the developed glass-ceramics are multifunctional materials. In this work, we have shown that cobalt ions, as representatives of transition metal ions, do not enter the crystalline phase of yttrium orthoniobate, but selectively enter crystals of the β-quartz ss. According to our previous studies [6,7] rare-earth ions, such as Er³⁺, Yb³⁺, and Eu³⁺, do not enter the structure of the β-quartz ss, but form their own phase of rare-earth orthoniobate or selectively enter the structure of yttrium orthoniobate [6,7]. By this means, rare-earth orthoniobates play the dual role of nucleating agents and active crystalline phases containing rare-earth ions. Thus we demonstrated the development of transparent glass-ceramics with near zero thermal expansion and with a selective doping of rare-earth and transition metal ions into different crystalline phases. These materials are promising for photonic applications.

4. This can be more interesting by studying the doping effect and investigating some others properties as ferroelasticity.

Thank you very much for your kind suggestion. We are planning to study these properties in our future works.

Thank you very much indeed for your kind suggestions that helped us to improve the quality of the manuscript.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors describe the mechanism of phase transformation mechanism on lithium-contained glass-ceramic which occurs during cooling process instead of high temperature heating. This finding is important to the field and can give a great potential on sharping the readers' thoughts on how to control the synthesis process smartly. Therefore, based on the quality and the scope of the manuscript, I recommend it to be published on MDPI Ceramics after minor revision. My comments are as follow:

1. The authors in the introduction part need to clarify which YNbO4 is selected instead of other earth-rare compounds. What are the advantages and disadvantages of YNbO4.

2. In the method section, the authors need to give more detailed information/synthesis process on the heat treatment was conducted, by providing information such as heating rate, how long the annealing process was, and under which atmosphere the experiment was conducted (Ar or N2 or air or O2). In addition, the authors also need to mention how the glass was quenched to make it transparence. 

3.  The authors also need to mention where the raw materials were bought from and the ceramic crucible.

4. In Figure 7d, why do the authors attribute the rod-like morphology to Beta spodumene ss. It looks like some extrusion happened. EDX mapping can potentially help to make a clearer distinction and help clarify what the authors mentioned. EDX mapping is highly encouraged to perform. 

5. The authors need to explain why heating/cooling rate of 30C/min was chose. Otherwise that can be just a random number. 

6. In Figure 5, the peak at 806 cm-1 became pretty sharp when treated at 1000C for 6hrs. The authors need to explain why that is the case. 

English is good to be understood. 

Author Response

We are very grateful to Reviewer 3 for valuable comments and suggestions that help us to improve the manuscript, and answer them point by point.

The authors describe the mechanism of phase transformation mechanism on lithium-contained glass-ceramic which occurs during cooling process instead of high temperature heating. This finding is important to the field and can give a great potential on sharping the readers' thoughts on how to control the synthesis process smartly. Therefore, based on the quality and the scope of the manuscript, I recommend it to be published on MDPI Ceramics after minor revision. My comments are as follow:

1. The authors in the introduction part need to clarify which YNbO4 is selected instead of other earth-rare compounds. What are the advantages and disadvantages of YNbO4.

Thank you very much for your question. As it was written in Introduction, “In this study, yttrium was taken as a representative rare-earth ion. Yttrium orthoniobate is distinguished by the combination of promising luminescent, chemical and mechanical properties. It is a self-activated X-ray phosphor [8] widely used in X-ray medical techniques. It is also a promising optical host for rare-earth ions because yttrium ions are easily replaced by other rare-earth ions [7,9-16].” We made the following explanation that is more detailed:

In this study, yttrium was taken as a representative rare-earth ion, and yttrium orthoniobate was chosen as a model rare-earth orthoniobate. Yttrium orthoniobate is a promising optical host for rare-earth ions because yttrium ions are easily replaced by other rare-earth ions in any proportion [7,9-16]. It should be noted that yttrium orthoniobate itself is distinguished by the combination of promising luminescent, chemical and mechanical properties. It is a self-activated X-ray phosphor [8] widely used in X-ray medical techniques. We believe the regularities of phase transformations found by the example of yttrium orthoniobates crystallized in LAS glass-ceramics are the general regularities that can be extended to other rare earth orthoniobates crystallized in LAS glass-ceramics.

2. In the method section, the authors need to give more detailed information/synthesis process on the heat treatment was conducted, by providing information such as heating rate, how long the annealing process was, and under which atmosphere the experiment was conducted (Ar or N2 or air or O2). In addition, the authors also need to mention how the glass was quenched to make it transparence. 

Thank you very much for your inquiry. The following correction was made:

The glass was melted in air in a home-made crucible made of quartz ceramics at 1560 °C for 4 h with stirring, poured onto a metal plate and annealed at 620 °C for 1 h. Then the furnace was switched off and the glass was cooled down with the annealing furnace. The transparent violet-blue colored initial glass was heat-treated in isothermal conditions from 700 to 1350 °C for 6 h by the two-stage heat-treatments with the first hold at 700 °C for 6 h. The heating rate was 5 °C per minute.

3. The authors also need to mention where the raw materials were bought from and the ceramic crucible.

The reagent grade raw materials were supplied by Nevareactive, Saint Petersburg, Russia. The batch weight was 300 g. The glass was melted in air in a home-made crucible made of quartz ceramics at 1560 °C for 4 h with stirring, poured onto a metal plate and annealed at 620 °C for 1 h.

4. In Figure 7d, why do the authors attribute the rod-like morphology to Beta spodumene ss. It looks like some extrusion happened. EDX mapping can potentially help to make a clearer distinction and help clarify what the authors mentioned. EDX mapping is highly encouraged to perform. 

According to your advice, the results of the EDX analysis are provided:

The microstructure and elemental composition of glass-ceramic prepared by the heat-treatment at 700 °C + 1350 °C were evaluated by SEM coupled with EDX analysis. The results are shown in Figure 8(a–c). It should be mentioned that the lithium content cannot be determined by the EDX method, and the content of all other elements was normalized to 100%.

Figure 8. SEM-EDX analysis of the etched surface of the glass-ceramic prepared by the heat-treatment at 700 °C + 1350 °C. The duration of each hold is 6 h: (a) SEM image; (b) EDX spectrum, point A; (c) EDX spectrum, point B.

According to Figure 8b and Table 2, bright round-shaped crystal denoted by the number 1 in Figure 8a, consists of yttrium, niobium and oxygen ions. We also made EDX analysis of the other round-shaped particles of different sizes and found that they have a very similar elemental composition. Aluminum and silicon ions that appear in the EDX spectrum in Figure 8b, probably come from the surrounding matrix and from rod-like crystals located nearby (we should bear in mind that aluminum oxide is more resistant to hydrofluoric acid that silica). Figure 8c presents the results of the EDX analysis of the material from the point 2 in Figure 8a. The point 2 was chosen as a place where a number of rod-like crystals is located, see Figure 8a. This composition is enriched in aluminum and silicon and depleted in yttrium and niobium as compared with the composition in the point 1.

Table 2. Compositions of the points 1 and 2 on the etched surface of the glass-ceramic prepared by the heat-treatment at 700 °C + 1350 °C (see Figure 8a).

5. The authors need to explain why heating/cooling rate of 30C/min was chose. Otherwise that can be just a random number. 

Thank you for your question. First, we used the standard heating/cooling rate of 10 °C/min and we did not notice any feature upon cooling. With the aim to reveal the possible effect of the tetragonal – monoclinic phase transition upon cooling, we increased the heating/cooling rate to 30 °C/min and succeeded to record the inflection point at temperature of 547 °C, which corresponds well to the temperature of the tetragonal – monoclinic phase transition. We have written it in the following way:

Section 2.2. Characterization, 2.2.1. Thermal Analisys. “Two sets of measurements were conducted, employing heating and cooling rates of 10 and 30 °C/min, respectively.” In the Results section, it is written, “The first set of DTA measurements was conducted using conventional heating and cooling rates of 10 °C/min. … No significant effects were observed during the cooling process. The second set of DTA measurements was performed at a higher heating and cooling rate of 30 °C/min. …The DTA curve collected on cooling showed an inflection point at temperature of 547 °C.”

To underline the significance of this finding, the following sentence is added: To underline the significance of this finding, the following correction is made: With the aim to reveal the possible thermal effect of the tetragonal – monoclinic phase transition upon cooling, we increased the heating and cooling rate, and the second set of DTA measurements was performed at a heating and cooling rate of 30 °C/min.

6. In Figure 5, the peak at 806 cm-1 became pretty sharp when treated at 1000C for 6hrs. The authors need to explain why that is the case. 

We believe that all peaks in the Raman spectrum of the glass-ceramic prepared by the heat-treatment at 1000 °C are narrow due to crystallization of rather large yttrium orthoniobate crystals with tetragonal and partly with monoclinic structure.

Thank you very much indeed for your comments and inquires that helped us to improve the quality of the manuscript.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

The authors have respond to the major comments,

Indeed, for the sentence of " studied for the first time" should be placed in the convenient position to indicates the novelty as the in situ study of phase transformation of these materials.

Reviewer 3 Report

Approve for acceptance. 

Need a moderate English polishing