Review Reports

Round 1

Reviewer 1 Report

The manuscript by Kovalev et al. reports citrate-based formation of the SrxBa(1-x)Fe12O19 ferrites and their basic structural and magnetic properties. The methods used and overall data looks good. However, I'm afriad that this work does not report anything new. Especially:

1. Formation of the different ferrites, including hexaferrites, through citrate method is well-known,

2. Formation temperature of 700C is relatively low compared to standard ceramic methods, but it is known for a wet-chemical reactions (for example DOI: https://doi.org/10.1021/jp068249b, https://doi.org/10.1016/j.ceramint.2013.12.068, https://doi.org/10.1016/j.jallcom.2005.08.024),

3. Substitution of Ba by Sr and formation of the corresponding solid solution is no new information (especially, since both BaFe12O19 and SrFe12O19 are two probably best known hexaferrites with a very analogical properties),

4. There are no unexpected changes in the structural and magnetic properties of the subistuted materials and mostly, they just follow the composition between the SrM and BaM. Generally, the changes are also relatively small,

5. Creation of the multi-domain structures and lowering of coercivity for the materials sintered at 1400C, with a grains > 1 microm, is expected as reported by multiple works e.g. for the pure BaFe12O19.

Also, up to the IR spectroscopy, analysis/discussion of the presented results is minimal, even without refering to the known data for the BaM/SrM (there is literally no reference on the pages 3-9, despite describing 5 different measurements). On the other hand, description of the THz/IR spectroscopy was quite thorough, however I must note that following the text with respect to actual presented data was hard and not obvious. For example, Authors state that:
"Since Sr2+ and Ba2+ ions randomly occupy one crystallographic position without any signs of ordering (Table 1), this means an increase in structural disorder, primarily in the system of (Sr,Ba)O12 anticuboctahedrons. A clear manifestation of the effect of structural disorder is the pronounced broadening of the lines corresponding to low-frequency oscillations (? < 280 cm-1) and the enhancement of mode damping observed in the THz and IR spectra of the samples with intermediate x(Sr2+) concentrations (0 < x < 1), especially with equal contents of barium and strontium (x = 0.5)."

From which I understand that this broadening is a feature characteristic for the subsituted materials and achives some maximum disorder for x=0.5. However, in the Figure 13 it is no obvious to observe these effects. Yes, there seems to be general broadening and damping, relative to pure SrM, but the most broad/damped singla appear to be for BaM? And certainly, it is not clear that for the x=0.5 there are generally borader/weaker signals than for x=0.3 and x=0.7. So, is it not just systematic change between the features characteristic for SrM and BaM, again depending on the composition? I believe that Authors should highlight these features better if such description was made.

Ultimately, I believe that in this form, there is no particular reason to publish this manuscript.

Some changes, that I could reccomend for the authors (including some minor errors and obvious drawbacks that I have noticed):

1. Please, describe the preparation procedure in more detail. For example, what were the amounts and concentrations of the substrates used for synthesis?

2. Was EDS analysis performed only for the SrM? Why not for the subsituted materials? Magnetic properties will depend also on the detailed composition. Furthermore, what was the composition obtained from the EDS? Giving that the Ms values of all materials are lower that theoretical (which is typical), there might be some non-stoichiometry (agaian, typical). How close to the perfect stoichiometry are those materials?

3. Authors state that: "The Tc is increasing with x, that probably related to shorter interatomic distances." I believe that this relation could be discussed a bit deeper, especially, since the Authors have performed detailed XRD analysis. What is an actual relation between these two?

4. I would reccomend determination of the effective magnetic anisotropy from the hystereses, compare it to the literature and comment if there are any differences upon the Sr/Ba substitution.

5. Analogically, the M(T) data can be analysed with the respect to the Bloch law (in a suitable region), which can be used to determine beta coefficients. Beta is known to be (roughly) proportional to the Curie temperature and due to my best knowledge, there not many works that report both experimental values, so this could be usefull for the potential readers. In any case, I would also recommend to the perform M(T) measurements for all samples the same (magnetized or demagnezited prior to measurements).

6. I would reccomend correction of the Figure 13, so the differences described in the text are easy to notice for the reader (or present it a different way?).

7. Lastly, in the conclusions Authors say that "Nanodispersed state of samples increase their effectiveness for creating a highly active catalyst or a pigment coating.". There is no data that show these materials as "highly active catalyst", so I would not state it as a fact.

 

 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this work, Ba–Sr hexaferrite ceramic powders were synthesized via two different procedures: the sol-gel method and the ceramic method. The most important features of the products were characterized by several techniques including XRD, SEM/EDS, DSC, THz, and IR. The study is well organized and the obtained results may be interesting for the scientific community. The manuscript could be accepted for publication after the following modifications:

- The English quality of the manuscript needs to improve thoroughly. Several short sentences could be merged to improve the readability of the text.

- As the authors stated, M-type hexaferrites have been synthesized via different methods. What is the novelty of the current work in comparison with the literature? For example, in a report by Shariff et al. (https://doi.org/10.1016/j.ceramint.2020.07.034), the sol-gel method has been used to synthesize similar compositions of the current work.

- The FullProf software was used for the Rietveld refining of all the XRD patterns. However, the crystallite size and the lattice strain were obtained by the Wiliamson-Hall method. Why this information did not obtain via the FullProf software?

- How many peaks were considered to plot the Wiliamson-Hall curve of each pattern? It seems the number of peaks is much higher than the indexed peaks of each pattern.

- Page 6, line 138: the authors state that “From this analysis average CSR size for all of our hexaferrites is 19.7 nm”. As can be seen from Fig. 3, the y-intercept of each plot is different from another one. So, how do all the products may show similar crystallite sizes?

- What is the ionic radius of Sr2+ and Ba2+? These values should be given in the manuscript. As the authors reported, by changing the Sr fraction up to 1, the variation of lattice parameters a, and c is low. Please explain this result base on ionic radius.

- Please replace Fig. 6 with colored ones, if possible.

- The English quality of the manuscript needs to improve thoroughly. Several short sentences could be merged to improve the readability of the text.

Author Response

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Reviewer 3 Report

Comments

The introduction needs to be rewritten with recent work.

In XRD plots, the Miller indices are not provided.

SEM images show agglomeration. Why?

The M-H curves offer unsaturated position. Have you used LAS plots for identifying paramagnetic portion?

In DSC curves, have you determined Tg, Tc, and Tm? Explain

Conclusions need to rewritten scientifically and technically.

Explain the motivation and novelty behind this work.

On what basis this work is better than the literature?

minor

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

In the revised version of the manuscript Authors have made severel improvements, in accordence with my comments, which I'm greatfull for. I think at this point the manunscript can be published, with last minor comments as follow:

1. "The Tc is increasing with x, that probably related to shorter interatomic distances due to substitution of larger Ba2+ ions by the smaller Sr2+ ions, that increases magnetic superexchange interactions between Fe sites."

Does the Auuthors mean that "Because Tc increase, superexchange interactions must also increase, probably due to smaller Sr radius" or "The Tc increases, probably due to the smaller Sr radius, which is known to increase superexchange interactions between Fe sites?" Please, clarify it and possibly add suitable references.

2. During the Keff determination, was the Ms value used in the calculations? Usually, it appears when transforming B to Keff. For example, in the reference [32] it appears as Keff = Ms*(15B/4)^(1/2). Because of this, it is important what is the exact unit of the field and Ms, before reporting Keff which can adopt multiple different units. In the revised manuscript, analogical expression for Keff eppar as equation (3), but in this case, the Ms is not there. Since the unit B is Oe^2 (in this manuscript), the unit of Keff would be Oe, based on equation (3), but the final value is presented in J/m3. I believe there might be some error here, somehwere, which might be the reason why final values are visibly lower than some other studies. In this regard, please verify if the calculated values are truly correct, before publishing final version.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper can be accepted.

Author Response

Thank you for your decision.