Unusual Breathing Behavior of Optically Excited Barium Titanate Nanocrystals

Round 1

Reviewer 1 Report

This manuscript is mainly interesting as the demonstration of another experiment at X-ray Free Electron Laser. The design and constructions of such laser has been a huge investment into the scientific infrastructure and the research at such facilities attracts some attention even if the scientific value and validity is a bit questionable. I believe the paper was submitted to "Crystals" with this sort if idea in mind. While the work probably deserves the publication at least in some form, still I have a few essential remarks:  

1. The abstract states "This unusual behavior can be explained as a piezoelectric response coupled with internal twinning of the nanocrystals".  Actually this is the first and the last time the authors say anything about piezoelectricity and twinning in their samples. There is nothing about this in the discussion nor in the other part of the manuscript.
2. The authors associate their observation with a rotation of nano- crystals. This conclusion is solely based on the fact that no radial displacement of Bragg peak was observed. However, this does not necessarily mean that such displacement must be associated with  rotation. For example, some shear deformation modes may also cause transverse displacement of Bragg peak. The conclusion about the origin of Bragg peak shift should be based on the analysis of collective displacement of several reflections and can hardly be done based on a single one only. 
3. Figure 1. The authors associate the changes of intensity with rotation around Z-Axis and the displacement of the peak with the rotation around X-Axis. In fact, any rotation that moves a reciprocal lattice node out of the Ewald sphere should cause the change of intensity. The only rotation which keeps the vector in the Ewald sphere is the rotation around the direction of a primary X-ray beam. And this is not X-Axis as it is shown in the figure. 
4. Figure 2 is pretty hard to follow. Better labeling (which graph correspond to X and Y direction and not only in the figure caption) would improve the readability of the paper. 
5. The same comment applies to Figure 4. The second and the third columns of these figures are hardly understandable and poorly explained. What is on the x and y axes? Most importantly, which information does it carry? Do the displacement figures (the third column) support the model of rotation of the crystal in any way?
6. Overall, I find the part of the paper, related to the Bragg Coherent Diffraction Imaging redundant because it does not seem to give any additional information.     

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Page 3 line 102-112

Is it possible to clarify what was exactly observed with a schematic representation? Or to show clear experimental results with indication to clarify exactly this paragraph?

Page 3, Figure 1 and its caption

it is not clear from the current information in Figure 1 and its Figure caption how the different displacements of reflections on the detector would be related to specific behaviour of the crystal itself. This should be explained more clearly.

Page 4, line 144

What does “it” refer to exactly?

Page 5, line 167 and Figure 4 and its caption on page 6

“Figure 4 shows the central slice of the 3D diffraction pattern”, more explanation should be given for those not familiar with specifically XFEL diffraction patterns: what are we looking at here?  Is it the 000 reflection?  Is it an extremely small mash up of all reflections?  How does this relate to the rings that were mentioned earlier in the paper as being the diffraction results?  Are the vague ring parts in these Fig.4 left column the rings along which the reflections shift during the experiment?  Again, a clear scheme and clear labels linking all things mentioned in the text to the different parts on the figures and adding where necessary new figures would improve the paper a lot.

Do the axis tick marks have any meaning on Figure 4 left column? What are the units for these axes?

Page 7, line 186

“Our XFEL experiment clearly shows that rotations of the BTO nanocrystals are induced by laser excitation”, on the contrary, the paper will only be clear to those doing exactly the same type of experiments, not for any crystallographer doing any other (diffraction) technique.  All clarifications mentioned above could help, though.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors responded to the comments and the paper can now be accepted for the publication in Crystals.

Reviewer 2 Report