Review Reports - Structural and Magnetic Investigations of the Novel Pyrophosphate Na7Ni3Fe(P2O7)4

Round 1

Reviewer 1 Report

The manuscript entitled "Structural and Magnetic Investigations of the Novel Pyrophosphate Na7Ni3Fe(P2O7)4" describes the synthesis, crystal structure determination, and magnetization measurements for the novel compound with the formula placed in the title of the manuscript. The motivation for the study is clear, and the methodology and results are meaningful and well-described.

I find however significant flaws in the crystallography part that prevent me from recommending accepting the manuscript right away:

1. There are no results of crystal structure validation. It is imperative to provide cif-file with check-cif results together with the submission. It would be also nice to deposit this crystal structure to ICSD. I would also like to see the results of bond-valence calculations for each atom in the structure as a part of structure validation.

2. LeBail fit is not sufficient to prove the agreement between powder and single-crystal samples. The crystal symmetry is low, the unit cell is rather big. It is thus necessary to show that not only cell metrics fit the diffraction pattern (LeBail method), but also the structural model reasonably reproduces the powder intensities (Rietveld method).

After these two points are addressed, I will be convinced about the core of the work. A few minor points to fix afterward are:

1. Please provide the source and purity of the starting reagents (lines 73 -75)

2. Long tables with crystallographic data (Table1 - 4) can be moved to SI.

3. The text on lines 239 - 242 is just a repetition of what is written 122-125 and can be omitted.

4. Why do authors claim that the feature in magnetic susceptibility at 29 K is due to short-range order? To me, it looks more like ordering in one sublattice at 29K followed by another at 3.5K. The frustration ratio is then just slightly above 1.

5. Please provide the intermediate calculations for the theoretical value of magnetic moment (line 270)

6. Figure 5d. Please recalculate emu/g to emu/mol. Why the measurements were done only up to 1T?

7. lines 275-278 "This magnetic behavior can be attributed to the occurrence of a spin frustration, which decreases when a high magnetic field is applied because a higher field strength can direct more particles even at lower temperatures." This sentence is completely unclear. How direction of "particles" (which particles, by the way) is related to frustration? I presume the measurements are done on compacted powders.

The quality of English is sufficient.

Author Response

Reviewer 1: The manuscript entitled "Structural and Magnetic Investigations of the Novel Pyrophosphate Na7Ni3Fe(P2O7)4" describes the synthesis, crystal structure determination, and magnetization measurements for the novel compound with the formula placed in the title of the manuscript. The motivation for the study is clear, and the methodology and results are meaningful and well-described.

We would like to thank Reviewer 1 for the thoughtful and insightful feedback on our article and for many useful suggestions to improve the quality of our manuscript.

There are no results of crystal structure validation. It is imperative to provide cif-file with check-cif results together with the submission. It would be also nice to deposit this crystal structure to ICSD. I would also like to see the results of bond-valence calculations for each atom in the structure as a part of structure validation.

Response: We thank the reviewer for the suggestions regarding the crystal structure validation. We have included the cif-file with check-cif results in our submission. Furthermore, we have deposited the Crystallographic data as a cif file in the Data Center with the CSD number 2264902. Additionally, the results of the bond-valence calculations are in line with the structural model and can be found in the Supporting Information (SI), Table S2. We have updated the revised manuscript accordingly.

LeBail fit is not sufficient to prove the agreement between powder and single-crystal samples. The crystal symmetry is low, the unit cell is rather big. It is thus necessary to show that not only cell metrics fit the diffraction pattern (LeBail method), but also the structural model reasonably reproduces the powder intensities (Rietveld method).

Response: We thank the reviewer for emphasizing the importance of thoroughly validating the agreement between powder and single-crystal samples in our study. To address this concern, we have conducted an additional analysis using the Rietveld refinement method. The results of the Rietveld refinement showed satisfactory R values and good agreement between the observed and the calculated powder XRD patterns. We have added the results in the revised manuscript (lines 114-115, 196-206) including Fig. 1a and Tables S5-S7 in SI.

After these two points are addressed, I will be convinced about the core of the work. A few minor points to fix afterward are:

Please provide the source and purity of the starting reagents (lines 73 -75).

Response: We thank the reviewer for this important comment. We have provided the source and the purity of the starting reagents in the revised manuscript.

Long tables with crystallographic data (Table1 - 4) can be moved to SI

Response: As suggested by the reviewer, we have moved tables 1-4 to the Supplementary Information (SI).

The text on lines 239 - 242 is just a repetition of what is written 122-125 and can be omitted.

Response: We thank the reviewer for this comment. To avoid repetition, we removed the repetitive lines 239-242 in the revised version of the manuscript.

Why do authors claim that the feature in magnetic susceptibility at 29 K is due to short-range order? To me, it looks more like ordering in one sublattice at 29K followed by another at 3.5K. The frustration ratio is then just slightly above 1.

Response: We thank the reviewer for bringing up this important question. Regarding the observed feature in magnetic susceptibility at 29 K. It does indeed appear more consistent with magnetic ordering in one sublattice followed by another at 3.5 K. We have updated the revised manuscript accordingly.

Please provide the intermediate calculations for the theoretical value of magnetic moment (line 270)

Response: We thank the reviewer for this suggestion. We have provided the intermediate calculations for the theoretical value of magnetic moment in the revised manuscript.

Figure 5d. Please recalculate emu/g to emu/mol. Why the measurements were done only up to 1T?

Response: We thank the reviewer for this comment. In the revised version of the manuscript, we have calculated the values in emu/mol. For completeness we have performed the M vs H measurements up to 9T which is the max limit in our instrument. It is found that with increasing the field up to 9T the M-H curve at 2 K takes S shape indicating the AFM ordering. We have updated Figure 5d and the revised manuscript accordingly.

lines 275-278 "This magnetic behavior can be attributed to the occurrence of a spin frustration, which decreases when a high magnetic field is applied because a higher field strength can direct more particles even at lower temperatures." This sentence is completely unclear. How direction of "particles" (which particles, by the way) is related to frustration? I presume the measurements are done on compacted powders.

Response: We appreciate the reviewer's feedback regarding the clarity of the sentence in question. To avoid confusion, we have corrected the sentence to ‘’This magnetic behavior can be attributed to the occurrence of a spin frustration, which decreases when a high magnetic field is applied, since a higher field strength can direct more magnetic spins even at lower temperatures.’’ The revised manuscript is updated with this sentence (lines 293-295).

Reviewer 2 Report

The article is written at a high level and contains interesting results.

Author Response

We would like to thank Reviewer for the careful reading and nice feedback on our paper.

Reviewer 3 Report

The presented manuscript describes preparation of the novel compound Na7Ni3Fe(P2O4)4 in form of crystals and single phase powder and its characterization by means of structural analysis, SEM, FT-IR and Raman spectroscopies and description of magnetic behavior. I think that manuscript will be sounds much better if authors may compare their compound with composition close one Na3.12 Fe2.44(P2O7)2 described in J. Angenault, J.C. Couturier, M. Quarton, and F. Robert. Structure of Na3.12Fe2.44(P2O7)2. European Journal of Solid State Inorganic Chemistry, 32:335–343, 1995. The both structures look similar, but described compound contains one position occupied simultaneously by Fe and Ni, whereas in the literature compound Fe share positions with sodium. May authors comment why some sodium atoms in Na7Ni3Fe(P2O4)4 crystal structure show very elongated thermal ellipsoids?

Author Response

Reviewer 3: The presented manuscript describes preparation of the novel compound Na7Ni3Fe(P2O4)4 in form of crystals and single phase powder and its characterization by means of structural analysis, SEM, FT-IR and Raman spectroscopies and description of magnetic behavior. I think that manuscript will be sounds much better if authors may compare their compound with composition close one Na3.12 Fe2.44(P2O7)2 described in J. Angenault, J.C. Couturier, M. Quarton, and F. Robert. Structure of Na3.12Fe2.44(P2O7)2. European Journal of Solid State Inorganic Chemistry, 32:335–343, 1995. The both structures look similar, but described compound contains one position occupied simultaneously by Fe and Ni, whereas in the literature compound Fe share positions with sodium. May authors comment why some sodium atoms in Na7Ni3Fe(P2O4)4 crystal structure show very elongated thermal ellipsoids?

We would like to thank the Reviewer for the efforts and for the constructive comments that have greatly contributed to the improvement of our work.

Response: We would like to thank Reviewer for providing valuable comments. In the revised manuscript we have compared the structure in this work with the isostructural one Na3.12Fe2.44(P2O7)2. We have also compared the lattice parameters of some isostructural pyrophosphates in Table 1. Regarding the relatively high displacement parameters of Na1 and Na2 compared to Na3 and Na4 is by the fact that both Na1 and Na2 sites are more distorted that Na3 and Na4 sites which explains their elongated displacement parameters. The calculated distortions for Na sites are given in Table S4. The revised manuscript is updated with this discussion (lines 144-145 and 173-179).

Reviewer 4 Report

This article discusses the synthesis of a new sodium-nickel-iron pyrophosphate Na7Ni3Fe(P2O7)4 in two different forms, single crystal and powder, as well as its structural characteristics and magnetic properties. Indeed, the recent focus has been on pyrophosphate-based materials due to their rich crystal chemistry and outstanding physical and chemical properties. In this regard, the work is relevant. Na7Ni3Fe(P2O7)4 pyrophosphate was analyzed by X-ray diffraction, SEM, FTIR, and Raman spectroscopy. It is shown that it crystallizes in the space group P1̅ of the triclinic system, and its structure has an open framework with distinct tunnels containing Na+ cations. Studies of the structure of trophophosphate look convincing. But the question arises about the results of the study of magnetic properties. The authors write: "Magnetic measurements revealed the presence of a distinct long-range order state occurring at a temperature of about 4.5 K, preceded by short-range AFM ordering at about 29 K." However, the sample of curves M(H) shown in Figs. 5d does not have the shape of hysteresis loops. The authors comment on this thus: "... the absence of hysteresis indicates a dominant antiferromagnetic ground state." But this is unconvincing and disputable; this form of the M(H) dependence is rather characteristic of paramagnets, and not of antiferromagnets.

Author Response

Reviewer 4: This article discusses the synthesis of a new sodium-nickel-iron pyrophosphate Na7Ni3Fe(P2O7)4 in two different forms, single crystal and powder, as well as its structural characteristics and magnetic properties. Indeed, the recent focus has been on pyrophosphate-based materials due to their rich crystal chemistry and outstanding physical and chemical properties. In this regard, the work is relevant. Na7Ni3Fe(P2O7)4 pyrophosphate was analyzed by X-ray diffraction, SEM, FTIR, and Raman spectroscopy. It is shown that it crystallizes in the space group P1̅ of the triclinic system, and its structure has an open framework with distinct tunnels containing Na+ cations. Studies of the structure of trophophosphate look convincing. But the question arises about the results of the study of magnetic properties. The authors write: "Magnetic measurements revealed the presence of a distinct long-range order state occurring at a temperature of about 4.5 K, preceded by short-range AFM ordering at about 29 K." However, the sample of curves M(H) shown in Figs. 5d does not have the shape of hysteresis loops. The authors comment on this thus: "... the absence of hysteresis indicates a dominant antiferromagnetic ground state." But this is unconvincing and disputable; this form of the M(H) dependence is rather characteristic of paramagnets, and not of antiferromagnets.

We would like to thank the Reviewer for providing valuable such insightful comments that have greatly contributed to the improvement of our work.

Response : We agree with the reviewer that linear M(H) is characteristic of paramagnets, however a close look to the M-H curve at 2K showed imperfect linear shape. To clarify this, we have remeasured M vs. H up to 9 Tesla and we found that the M-H curve at 2 K takes S-shape which supports the AFM ordering. We have updated Figure 5d and the revised manuscript accordingly. We appreciate the reviewer’s comment.

Round 2

Reviewer 1 Report

The authors have appropriately addressed the comments from the Reviewers, and the manuscript can be recommended for publication

The quality of English is appropriate.

Author Response

We would like to thank Reviewer for recommending the publication of the manuscript.

Reviewer 3 Report

I suggest that references style should be checked before publication.

Reference 27, lines 405-407. Do not capitalize all letters

Reference 34 line 421 looks strange. I think 'T.M. Project' is not the real name. It is better to cite as Materials project https://materialsproject.org/materials/mp-1011834

Also, reference 36, line 424. Do authors mean the book X-ray Metallography by A. Taylor, published by Wiley, 1961, pages 334-335?

Author Response

We thank the reviewer for the valuable comments. we have corrected the reference 27, 34 and 36 in the revised manuscript. Thanks.

Reference 27, lines 405-407. Do not capitalize all letters

We have revised the reference :
[27] J. Rodríguez-Carvajal, Magnetic structure determination from powder diffraction using the program Fullprof, in: Appl. Crystallogr., World Scientific, 2001: pp. 30–36. https://doi.org/10.1142/9789812811325_0005.

Reference 34 line 421 looks strange. I think 'T.M. Project' is not the real name. It is better to cite as Materials project https://materialsproject.org/materials/mp-1011834

We have revised the reference :
[34] Materials Data on Na2CoP2O7 by Materials Project, (2016). https://www.osti.gov/dataexplorer/servlets/purl/1328091.

Also, reference 36, line 424. Do authors mean the book X-ray Metallography by A. Taylor, published by Wiley, 1961, pages 334-335?

We have changed the reference to a book reference :

[36] H.P. Klug, L.E. Alexander, X-Ray Diffraction Procedures: For Polycrystalline and Amorphous Materials, 2nd Edition, 1974.

Reviewer 4 Report

The authors have made the necessary changes. I think the article can be published as it is.

Author Response

We would like to thank Reviewer for recommending the publication of the manuscript.