High-Pressure Phase Diagrams of Na2CO3 and K2CO3
, Altyna Bekhtenova 1,2, Sergey S. Lobanov 3, Anton Shatskiy 1,2, Anna Yu. Likhacheva 2,4, Dinara Sagatova 1,2, Nursultan Sagatov 1,2, Sergey V. Rashchenko 1,2,4, Konstantin D. Litasov 1,2, Igor S. Sharygin 5,6, Alexander F. Goncharov 7, Vitali B. Prakapenka 8 and Yuji Higo 9Round 1
Reviewer 1 Report
The authors explore the P-T diagram of two components: Na2CO3and K2CO3. For this, they use a combination of high pressure and high temperature techniques as well as density functional theory. The high point of the manuscript is the amount of work, which tries to constrain the crystallography of two important carbonate systems at high P-T. However, the manuscript severely lacks details, especially when it comes to the discussion of results and implications of the findings and how these would benefit the community. The paper would also benefit restructuring for better readability, especially the “Results and discussion” section. Below are a few points of concern as well as suggestions to improve the manuscript.
Major comments:
The structure of the text in general does not flow well and makes the overall reading difficult. I suggest restructuring the text (see comments below). An effort should also be made to improve the figures: phase indexing is missing from most diffraction patterns and makes noticing phase transitions difficult. Guides to the eye are always helpful! Also, the paper does not discuss any potential implications of the results. Globally, it lacks details and discussion of the results.
L.156: “substantial changesof γ-Na2CO3diffraction pattern”. Could you be more specific? In Figure S2, patterns at 13.1 GPa and above show changes at 2θ > 9º: changes in peak intensity and appearance of new peaks. Could that be due to appearance of a new phase, however minor? These changes should be discussed, even if they are not deemed relevant by the authors.
Also, the discussion regarding the discrepancy between MA and DAC results is completely missing. Why is it that you don’t see a phase transition in DAC while you see it in MA? Please elaborate.
The Na2CO3subsection of the Results and discussion section should be restructured for better readability. You may want to first describe your approach (use results of dft calculation on P63/mcm and P21/m structures to compare with experimentally obtained diffraction patterns), then describe the results and finally discuss these results.
L.197: “three high-pressure phase transitions are observed (Fig.4)”. It is not clear in Fig.4 where the different phases are, please label the patterns accordingly.
L.214: “Most of the peaks of α-K2CO3-IV diffraction pattern can be satisfactorily indexed with K2CO3-P21/m structure” and then “we cannot accept P21/m structure as the structure of the experimental phase IV”. Is it possible that you might also have a mixture of two phases with different but similar structures?
Minor comments:
L.67: “pressures up to 20 GPa”. L.157 you say that you performed DAC experiments up to 25 GPa.
L.136–139: Is this method used for all diffraction experiments or only for a subset of them.
L.139: “less than 0.1 GPa uncertainty”. Least-square fitting might give you less than 0.1 GPa of uncertainty for Au, however there are other sources of uncertainty that are intrinsic to experiments, which give higher uncertainties like pressure gradients in the sample chamber. I highly doubt that your overall pressure uncertainty is only 0.1 GPa.
L.157: Please consider correcting: “substantial changes inγ-Na2CO3diffraction patternsare not observed”.
L.158: “can be found in Supporting Information”. Please mention the figure numbers you are referring to in the text, in this case S1 and S2. Please do this for all other mentions to figures and tables from the supplementary information. There are many in the text and this will improve the readability of the manuscript.
L.159: “are consistent with theoretical predictions”. Are you referring to the theoretical predictions performed in this study or to a past study? Please mention it.
L.179: “Indexing of diffraction patterns […] is ambiguous” Please refer a figure.
L.183: “The obtained P-T diagram (Fig. 3)”. Please mention that this is for MA experiments.
L.184 & 206: “fusion curves”. I would rather use the term “melting” than “fusion”.
L.199: Can you mention the crystal structures/space groups of phase II and III of K2CO3? If not, you want to state that they have not been determined.
L.208: “according to theoretical predictions” Again, are you referring to calculations performed in this study?
L.211: “on atomic shifts does not exceeding 0.07Å.” This part of the sentence is very confusing. Please rephrase.
L.220: Please cite reference work for the calculations you mention here.
L.231: “The results of experiments (II) have been published in [50]”. This should have be mentioned in the Methods section from the beginning.
L.234: “the new phase”: which new phase are you referring to? Also, please explain how diffraction patterns between MA and DAC experiments differ.
L.237: “Due to the catalytic activity of K2CO3 in the mixture with methanol in biodiesel production” This sentence should be enhanced with more details as to why this is important.
Figure 4: Please index phases or peaks corresponding to different structures mentioned in the text (same for Fig. S1, S2). Also, the evolution of the position of the second Au peak with increasing pressure is not clear.
Figure 6: I suggest using another color than purple for Ne since it is difficult to distinguish well from red for Au. You might also want to mention what green corresponds to.
Figure 7: Please emphasize which peaks correspond to the new phase.
Figure S3: I would suggest using another color (other than the same blue used for LDA) for the average between LDA and GGA calculations.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The manuscript reports the phase diagrams of Na2CO3 and K2CO3 using X-ray diffraction combined with multi-anvil and diamond anvil cell techniques. It demonstrated the phase changes of both compounds by collecting and analyzing many XRD patterns at increasing pressures. Ab initio calculations have been conducted to predict the crystal structure.
Major comment:
The manuscript is missing the discussion and conclusion sections, which makes it look like an experimental report. There are many discussion points, such as difference between Na2CO3 and K2CO3 and the reason (ionic radi of the cations?). The manuscript needs to be significantly expanded to include those discussions after demonstrating the experimental results. Furthermore, the unidentified peaks observed in the XRD patterns for the new phase(s) should also be discussed.
Some specific comments:
Line 27 & 54: "which" here is not properly used (grammatically not correct).
Line 157-158, 177-178, 189-191, 203-204, 215-216, 234-236: References to supplementary figures references in those lines, such as "Fig.S1, Fig. S2", should be specified clearly. Otherwise, readers will have difficult time to refer to the relevant supporting information.
Line 179-182 & Fig.2: There are two main unidentified peaks at ~55 keV and 74 keV, which have quite high intensity. It should be better to discuss more about those peaks. gamma-Na2CO3 has an incommensurate modulated structure. So is it possible those unidentified peaks are related to modulation of the new structure?
Line 183-184: I do not understand what this sentence means. Was the phase diagram determined from the experimental data? It was not clear how the phase diagram is related to "smooth" fusion curve. This paragraph of one sentence certainly require significant expansion in order to show what the authors would like to infer.
Line 192-194: The results of equation of state fitting are shown, but there is no compression curve to demonstrate the quality of the fitting. A volume versus pressure plot should be included to demonstrate the structure change and the compressibility of each phase.
Fig. 4: It is really difficult for readers to understand the results from the XRD patterns. In the figure, most of the peaks are not identified or labeled. Are there any unidentified peaks?
Lien 214-220: Some intense peaks can not be indexed. It is stated that P21/m is not the structure. However, in the abstract, it sounds like P21/m is the right structure to explain the XRD patterns. Some detailed explanation is needed.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
This manuscript is dedicated to study of high-pressure transitions in solid Na2CO3 and K2CO3 phases using the multianvil and diamond anvil cell techniques. In my opinion the manuscript is original, provides interesting results, however some minor corrections are necessary before publication.
1) It is unclear which author relates to affiliation number 9 (Japan Synchrotron Radiation Research Institute)?
2) It is written in the abstract that two phase transitions have been observed for Na2CO3: first – at 12 GPa and the second one – at 26 GPa, however in the manuscript these phase transitions occurred at 13.5 and at 27GPa. Please correct it.
3) Please explain the LDA and GGA abbreviations.
4) Explain which phases indicate the blue and red reflexes in the Fig.1 and 2?
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The new version shows significant improvements and I recommend the manuscript for publication.
Reviewer 2 Report
The authors have adequately addressed the comments of the 1st round review. I would recommend publication of the manuscript in "Minerals".
