Effect of Segregation of Sc, Y and La Atoms on Prenucleation at the Liquid-Al/γ-Al2O3{1 1 1} Interfaces
Round 1
Reviewer 1 Report
Fang and Fan in this manuscript present a theoretical investigation of the prenucleation at the Al(l)/γ-Al2O3 interface with segregation of Sc, Y and La (nd1) atoms using an ab initio molecular dynamics simulation. A systematical investigation was done for the atomic ordering and chemical interactions at the Sc, Y and La segregated Al(l)/γ-Al2O3{1 1 1} interfaces. In general, the manuscript is well-written, and conclusions are supported by computed data. I suggest accepting the manuscript after a final perusal to correct typing and Grammatical errors, for example:
--- Lines 92-93: The subjective and verb are missing in the following sentence: “Thus, a hexagonal supercell with a = 11.37Å , c = 37.18Å .”
--- throughout the text: number and unit should be separated by a space, like 700.0 eV, etc…
Author Response
We thank Reviewer 1 for the constructive comments and suggestions. Here is a summary of our responses. The comments from the reviewers are labelled by C and the responses by R.
Reviewer 1
C: -- Lines 92-93: The subjective and verb are missing in the following sentence: “Thus, a hexagonal supercell with a = 11.37Å, c = 37.18Å.”
--- throughout the text: number and unit should be separated by a space, like 700.0 eV, etc…
R: We corrected the missing words in the sentence and add a space to separate the numbers and related units throughout this manuscript. Moreover, we improved the language in the revised manuscript.
Reviewer 2 Report
This paper describes a computational study of the role of Group 3 atoms Sc, Y, and La on prenucleation sites at the liquid Al/Al2O3 interface. They use a standard DFT approach with the PBE functional for the AIMD study. The work is good but the presentation needs improvement.
What temperature were the simulations done at? Give this in the computational section.
Define room temperature as 298 K in the text.
Do not use the term ‘nd1 atoms’. Use ‘Group 3 atoms’ as this is correct nomenclature.
The biggest issues with the paper are the figure captions. Define each (a), etc in a figure. It is very hard to read the Group 3 atom in the individual figures, for example. (a, (b, (c, etc, need to be defined in the figure captions for each figure
For figure 1, give the time the snapshot is obtained at.
Line 182 ‘nd1’ needs to ‘nd1 atoms’ and as noted above this nomenclature should be changed to Group 3 atoms.
At what time are the results in figure 3 obtained?
In figure 4, I have no idea how the structures correspond to the figures as the i), etc. are not consistent with the (a), etc.
Figure 5. At what time step? Also, see above about defining (a), etc.
The notes to Table 1 need to be converted to superscripts and put with the appropriate columns or values. I have no idea what goes with what.
Figure 6. What time step? Same for figure 7.
Equation (2). Remove ΔH as it is not needed. They also do not calculate the enthalpy. All they calculate is an electronic energy as shown in equation (3).
The calculated energies in table 2 are only good to two decimal places in eV at best for this method. How can they take energy differences between Eform aand EAl2O3 as the atoms differ? This is not correct as they cannot be subtracted from each other because the atom types are different. This a well-established issue with this type of formula unit approach. It is iused in the solid state physics community and is not correct in terms of the chemistry. This will change all of their discussion and conclusions. One cannot do this as the zero of energy is different for each metal. They can do an exchange reaction for the metals with respect to Al but this was not done. What was done is just completely incorrect.
Compare the radii of the +3 ions to the literature values usually used.
This paper needs major revision due to issues with the analysis of the energetics.
Author Response
Responses to the report
We thank reviewer 2 for the constructive comments and suggestions. Here is a summary of our responses. The comments from the reviewers are labelled by C and the responses by R in the attached file.
Author Response File: 
Author Response.pdf
Round 2
Reviewer 2 Report
This paper is substantially improved. The authors now show what they did for the energetics that I was concerned about. This is an improvement over what was in the paper and is the correct way to do it in part. It is not perfect but it should give a good semi-quantitative picture of the interface. I would be surprised if redox is taking place if al of the metal ions are +3 as that would mean that the redox is on the oxygen which I would be surprised to find as the spin states would have to change. Where would the electrons from the oxygen go if the metals do not change? One cannot look at coordination number to get redox states.
Author Response
We thank Reviewer 2 for the careful reading and efforts. We respond the comments.
Q1: This paper is substantially improved. The authors now show what they did for the energetics that I was concerned about. This is an improvement over what was in the paper and is the correct way to do it in part. It is not perfect but it should give a good semi-quantitative picture of the interface.
Re1: We thank Reviewer 2 for the positive comments on the improvements. Meanwhile, the meaning of the redox is described in the text. We consider that an extra semi-quantitative picture may not add much value to the readers.
Q2: I would be surprised if redox is taking place if al of the metal ions are +3 as that would mean that the redox is on the oxygen which I would be surprised to find as the spin states would have to change. Where would the electrons from the oxygen go if the metals do not change? One cannot look at coordination number to get redox states.
Re2: As shown in the manuscript, the Al(l)/Al2O3 interfaces consist of three parts: i) the substrates, in which both AL and O atoms exhibit ionic nature; ii) liquid away from the interfaces, in which the Al atoms are electronically neutral and exhibit metallic behaviour; iii) around the interfaces, the metal atoms are partially charged (Figure 7). We can consider region 2 as the reservoir of neutral metal atoms. In the redox reactions (Equation 4), the neutral La atom in the reservoir replace the oxidized Al ions near the interfaces and the reduced Al atoms move back to the reservoir. There is no charge of the valences of the outmost O ions and there is no change of electrons at the O sites. This corresponds well to Figure 7.
We agree with Reviewer 2 that one cannot look at coordination number to get redox states directly. This is because that the segregation of the less electronegative and larger sized Group 3 atoms causes local structural relaxation/reconstruction. However, the larger charge losses of the segregated Group 3 atoms/ions as compared to those Al at the same distances from the outmost O atoms (Figure 7) provide evidence of the redox reactions at the interfaces. The redox reactions for the less electronegative Group 3 atoms to reduce the more electron-attractive Al atoms at the interfaces follow the rules of Chemistry.
