Effect of Na–Sn Flux on the Growth of Type I Na8Si46 Clathrate Crystals
Round 1
Reviewer 1 Report
The manuscript " Effect of Na–Sn Flux on the Growth of Type I Na8Si46 Clathrate Crystals" investigated the effect of Na–Sn flux on the growth of type I Na–Si clathrate crystals under various flux conditions. The flux-assisted method proposed in this study is suitable for the stable generation of a single phase of type I Na–Si clathrate crystals. I recommend acceptance for publication after major revisions. And the following issues should also be addressed.
1. The authors successfully grew the type I Na–Si clathrate crystals using Na-Sn flux method. The Na–Si clathrate crystals can also crystalize in type II structure with Fd3Ì…m space group. What is the effect of the Na-Sn flux for the growth of type I Na–Si clathrate crystals?
2. The Seebeck coefficient of Na–Si clathrate crystals is important for practical applications. The thermoelectric characterizations of Na–Si clathrate crystals prepared under various growth conditions should be provided if possible.
3. What are the advantages of Na-Si clathrates over other Si clathrates? Please give more discussion in Introduction.
4. The authors mentioned that the rate of Na in the utilized container can be expressed as: v = 1.77×10−4 + 0.58t×10−4 mol/h. However, the physical meaning of these formula parameters is confusing. Please give some detailed explanations.
Author Response
Response to Reviewer 1 Comments
Thank you for your kind comments.
Point 1: The authors successfully grew the type I Na–Si clathrate crystals using Na-Sn flux method. The Na–Si clathrate crystals can also crystalize in type II structure with Fd3Ì…m space group. What is the effect of the Na-Sn flux for the growth of type I Na–Si clathrate crystals?
Response 1: Type II Na-Si clathrate crystal is also grown using Na-Sn flux method. We have been reported the type II single crystal in the previous study (reference No. 11). In this experiment, only type I was formed and type II was not formed. It is not clear why type II is not formed, but it seems that the flux is working. In particular, it seems that controlling the evaporation of Na by using flux is a major factor. We think that the details of mechanism will be clarified by examining the effect of the flux for type II, so let me leave it as a future work. The effect of the flux in this study was added a little at the end of the text.
Point 2: The Seebeck coefficient of Na–Si clathrate crystals is important for practical applications. The thermoelectric characterizations of Na–Si clathrate crystals prepared under various growth conditions should be provided if possible.
Response 2: As you mentioned, the Seebeck coefficient is important for practical application of thermoelectric materials. We would like to know the thermoelectric properties of our single crystals. However, we don’t have the measuring equipment for the small crystals. The electric conductivity was already measured and it has been clear that the single crystal show the metallic conductivity in the previous study (reference No. 10). We will prepare the more large size of single crystal in the future, and we will measure the thermoelectric characterizations of Na–Si clathrate crystals. Therefore, measurement of properties should be an issue for the future works.
Point 3: What are the advantages of Na-Si clathrates over other Si clathrates? Please give more discussion in Introduction.
Response 3: The Na-Si clathrate itself does not have an advantage, but it is valuable as a precursor to the guest-free Si clathrate. guest-free Si clathrate is promising as a next-generation solar cell material. It is generally produced by extracting Na from Na-Si clathrate. Si single element clathrate is expected to be applied in various fields, and bulk crystals of its precursor Na-Si clathrate are indispensable for device fabrication. I mentioned the guest-free Si clathrate with references in the introduction.
Point 4: The authors mentioned that the rate of Na in the utilized container can be expressed as: v = 1.77×10−4 + 0.58t×10−4 mol/h. However, the physical meaning of these formula parameters is confusing. Please give some detailed explanations.
Response 4: We are sorry we couldn’t explain this meaning well and confused you. As you say, this expression is certainly confusing, so we revised this expression and explained the case of x = 0.5.
Reviewer 2 Report
The manuscript by Morito et al. discusses the effect of flux on crystal growth of type-1 Na-Si clathrates. The paper deals with detailing of the flux technique developed by authors earlier. The authors succeeded to grow relatively large clathrate crystals without secondary phases by optimization of the flux concentration, temperature and gas pressure. This is an appropriate result worth to be published in Crystals. However, the presentation of the results should be improved. This concerns not only the language but also the style of the manuscript. Some passages demand additional efforts to understand what authors wanted to say. On the other hand, there are several unnecessary repetitions in the text. The text should be more logically structured. Therefore, I recommend that a major textual revision be warranted. Moreover, I have some particular questions that should be cleared in the manuscript:
- Does clathrate dissolves in Na-Sn flux completely (a real Na-Si-Sn solution)? Depending on this, the nucleation problem may be an issue.
- Line 77. “The pressure inside the container was reduced to appr. 10^4 Pa using a dry pump”. What is the content of this atmosphere? Is it mainly Argon, Na vapour or what?
- Authors use the term evaporation rate once as a fraction of evaporated Na expressed in %, based on the reaction equation. In the next sentence, this is already a kinetic value expressed in mol/h.
- In line 203, 798 K is given as the melting point of Na9Sn4, whereas in Line 209 751 K.
Author Response
Response to Reviewer 2 Comments
Thank you for your kind comments. As you mentioned, it is difficult to understand this article because the discussion for the effect of flux and the experiment synthesized without using flux were mixed up. We have reconstructed the entire text.
Point 1: Does clathrate dissolves in Na-Sn flux completely (a real Na-Si-Sn solution)? Depending on this, the nucleation problem may be an issue.
Response 1: As you mentioned, the solubility of Na4Si4 precursor in Na-Sn flux is important point. Since we have not seen it directly, we cannot say that a complete solution has been prepared. However, in the previous work, the shape of Na4Si4 precursor changed like a melted shape by just heating not evaporation of Na. Therefore, it is assumed that the Na-Si-Sn solution is completely prepared in the present study. We will study the detail of solubility in this method in the future works.
Point 2: Line 77. “The pressure inside the container was reduced to appr. 10^4 Pa using a dry pump”. What is the content of this atmosphere? Is it mainly Argon, Na vapour or what?
Response 2: In the present study, the pressure inside the container filled with Ar gas was reduced to approximately 104 Pa. We added information of Ar gas.
Point 3: Authors use the term evaporation rate once as a fraction of evaporated Na expressed in %, based on the reaction equation. In the next sentence, this is already a kinetic value expressed in mol/h.
Response 3: We are sorry we couldn’t explain this meaning well and confused you. As you say, this expression is certainly confusing, so we revised this expression and explained the case of x = 0.5.
Point 4: In line 203, 798 K is given as the melting point of Na9Sn4, whereas in Line 209 751 K.
Response 4: The value of 798 K is incorrect. The correct value was 751 K. We revised this value. Thank you for pointing out.
Round 2
Reviewer 1 Report
The manuscript can be accepted in the current version.
