Study of New Infrared Non-Linear Optical Crystal BaGa₄Se₇ Based on First Principle

Round 1

Reviewer 1 Report

The manuscript entitled "Study of new infrared non-linear optic crystal BaGa4Se7 based on first principle" presents an ab initio study of the electronic and optic properties of a relatively new material BaGa4Se7. The topic of the manuscript could have been interesting for the readers of the journal. However, the paper exhibits worrying signs of either insufficient rigor and attention to detail, or simply manipulating results. Let us consider, for instance, Section 3.1. The authors claim they used the BFGS method to optimize the geometry of BaGa4Se7, however, there is no BFGS method implemented in the VASP code they used. Let us assume they used an external BFGS-based optimizer and look at the results of the optimization shown in Table 1. In my whole career, I have never seen such an amazing agreement between experiment and theory (4 digits after the floating-point). This is clearly an instance of copy-paste and I refuse to believe this result until I see the VASP output files. In Section 3.2.1, the authors state that BaGa4Se7 is a direct bandgap material, with VBM and CBM located at the G-point. However, when I look in FIgure 2 and Table 2, I see that VBM and CMB are actually at the Z-point. In Section 3.3.1, the authors claim that BaGa4Se7 is a tetragonal biaxial crystal. I do not understand, how monoclinic BaGa4Se7 (space group Pc, #7) all of a sudden became tetragonal. Given this assessment, and especially the fact that already at the stage of the geometry relaxation the authors seem to have manipulated their results, which renders the rest of their results questionable, I do not recommend this paper for publication.

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript entitled “Study of new infrared non-linear optical crystal BaGa₄Se₇ based on first principle”. We have studied reviewer’s comments carefully and the paper was revised according to the referees’ comments as follows:

Author Response File: Author Response.docx

Reviewer 2 Report

• The style and character of the paragraph between line 77 and 79 is not identical to that of the rest of the other paragraphs.
• Line 30 : effect instead of effec ?
• In my opinion ; Talking about phosphorus compounds in the introduction is not justified because the material studied in this article is about of the chalcogenide materials.
• In the article no indication concerning the synthesis mode of the material studied and the different characterization techniques used. Also no detail description of the crystal structure of BaGa4Se7 are given.

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript entitled “Study of new infrared non-linear optical crystal BaGa₄Se₇ based on first principle”. We have studied reviewer’s comments carefully and the paper was revised according to the referees’ comments as follows:

Author Response File: Author Response.docx

Reviewer 3 Report

In the submitted MS the authors provide a systematic study of the NLO optical properties BaGa4S7 within the framework of Density functional theory. The authors used more than one functionals  to draw their conclusions which are based, according to the reviewer's opinion on solid numerical data and a good analysis of the results.  The system proposed seems to be prominent for photonic applications.

In conclusion, the submitted MS is suitable for publication.

Comments

A. The authors should make an effort to improve the figures visualizing the bands of BeGa4Se7 and focus only on the valence region of this system bearing in mind that the NLO properties of any systems depends on its valence electrons.

B. It would be very useful if the authors provide and discuss the projected density of states. This info is of great importance because it might give some qualitative info about electronic transitions that are responsible for the NLO behavior of this material.

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript entitled “Study of new infrared non-linear optical crystal BaGa₄Se₇ based on first principle”. We have studied reviewer’s comments carefully and the paper was revised according to the referees’ comments as follows:

Author Response File: Author Response.docx

Reviewer 4 Report

Xie Huang , Rong Dai , Lei Zhang , Jing Ning , Fuchun Zhang , and
Junfeng Yan have submitted a manuscript entitled “Study of new infrared non-linear optical crystal BaGa4Se7 based on first principle” to the journal MDPI Crystals.

The research design is appropriate. The methods are adequately described. The results are clearly presented. Discussion of data and conclusions are adequately supported by the results.

English language and style are minor spell check required.

I do not detect plagiarism and I do not detect inappropriate citations.

In general, I do not see any ethical issues along the manuscript.

However, I am afraid that two references:

1)
Havva Bogaz Ozisik, Haci Ozisik & Engin Deligoz (2017) A new quaternary semiconductor compound (Ba2Sb4GeS10): Ab initio study, Philosophical Magazine, 97:8, 549-560, DOI: 10.1080/14786435.2016.1269967

2)

A.P. Yelisseyev, S.I. Lobanov, P.G. Krinitsin, L.I. Isaenko,

The optical properties of the nonlinear crystal BaGa4Se7,

Optical Materials,

Volume 99,

2020,

109564,

ISSN 0925-3467,

https://doi.org/10.1016/j.optmat.2019.109564.

Might jeopardize the novelty of this manuscript. While the second paper has been cited by the authors (Reference 11 in the submitted manuscript), the first one is not cited and the message is very similar.

I would suggest the authors to better describe the state-of-the-art and clarify what is the advancement of the submitted manuscript.

Other comments:

• Figure 6b: y-axis label should be “Reflection” and not “Refractive”
• How can the authors study reflection and absorption up to >21 eV if they do not show the complex dielectric function up to that energy?
• What is the confidence of the optical transitions up 21 eV, where the occurrence of transitions between many states is reasonably foreseen?

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript entitled “Study of new infrared non-linear optical crystal BaGa₄Se₇ based on first principle”. We have studied reviewer’s comments carefully and the paper was revised according to the referees’ comments as follows:

Author Response File: Author Response.docx

Reviewer 5 Report

In the current work authors provide a throughout theoretical investigation of electronic and optical properties of BaGa4Se7, which previously was proved experimentally to be infrared NLO material. The language of the article is clear and understandable.

However there are several problems that need to be addressed:

1. Could you please develop your thoughts on the practical significance of your work, since at the moment the main conclusion that BaGa4Se7 is infrared nonlinear optical crystal material, which was already proved by numerous experimental works. So in current form your research is reproduce experimental result, I sujet you focus more on predicted properties of this material, since your computational methods have good correlation with experimental data.
2. Could you comment in details why during the DFT optimization the relative errors for all geometrical parameters (even for the angles) was zero?
3. Please add  in the Computation details information about PBE, HSE06, HSE03 B3LYP, and PBE0 methods with all necessary literature references.

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript, we appreciate editor and reviewers very much for their positive and constructive comments and suggestions on our manuscript entitled “Study of new infrared non-linear optical crystal BaGa₄Se₇ based on first principle”. We have studied reviewer’s comments carefully and the paper was revised according to the referees’ comments as follows:

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I checked the response and the amendments the authors made. To me, they appear sloppy both in terms of the scientific depth and the language (numerous typos). The biggest concern for me still remains: in the description of the computational details, the authors provide such statements, that make me doubt their qualification. For instance, they did not respond to my comment on the absence of the BFGS method in the VASP code they used. They did not provide the VASP output to prove the lattice parameters they now put in response to my criticism. They claim to have used the 3x3x4 k-mesh while computing a material with the lattice parameters of a=7.8468 Å, b=6.8409 Å, c=15.5576 Å. This is against the best practices for sampling the Brillouin zone, which prescribes using fewer k-points for the longer directions since the corresponding vector in the reciprocal space is shorter. These are mistakes typical of novice to solid-state physics and to DFT calculations. Given that the authors make such crude mistakes already at the beginning, I have serious doubts about the rest of their work. So, my recommendation is: reject.

Author Response

Dear review：

Thank you very much for your reply.

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors have submitted a revised version of the manuscript entitled "  Study of new infrared non-linear optical crystal BaGa4Se7 based on first principle " to the journal of MDPI Crystals.

Together with the revised manuscript, the authors have provided a point-to-point response letter in which they have addressed all the questions raised by the referee.

In my opinion, the answers of the authors, and the subsequent changes and integrations in the manuscript, are proper.

For this reason, I would recommend publication of the manuscript in the journal.

Author Response

Dear review:

  Thank you very much for your reply and recognition.

Reviewer 5 Report

I satisfied with authors replies and with the changes were made. 

I have just few remarks:

1) Please use the same number of the significant digits when making comparison with experimental lattice parameters (Table 1).

2) Please use the same  number of the significant digits for other data as well (e.g Ev and Ec in Table 2 etc).

Author Response

Dear reviewers：

  We thank you very much for once again giving us an opportunity to revise our manuscript. We have revised the article. Please check it out.

Author Response File: Author Response.docx