Comparison of the Physical Properties and Electronic Structure of Nb2B3 and Ta2B3
Jakub Kaczkowski
Round 1
Reviewer 1 Report
Although the paper does not contain any technical sophistication the paper is of interest for readers.
There are few omissions or mistakes. For instance, the Fermi energies are given in electrons/eV, which is either wrong or confusing. The Fermi energies would have had a greater impact if the energy bands were depicted. I would ask the authors to address this issue.
Also the authors state :"As an important family of TMBs, it is a meaningful work to study the physical properties of 5d-series transition metal borides with low B content." On the other hand their crystals contain rather a large content of B: (Nb_2 B_3) and (Ta_2 B 3). Please clarify this aspect!
To me the English is not bad.
Author Response
Point 1: There are few omissions or mistakes. For instance, the Fermi energies are given in electrons/eV, which is either wrong or confusing. The Fermi energies would have had a greater impact if the energy bands were depicted. I would ask the authors to address this issue.
Response 1: The unit of Fermi energy is electrons/eV, which is obtained from the Density of States. The original plot is given as follows.
Point 2: Also the authors state :"As an important family of TMBs, it is a meaningful work to study the physical properties of 5d-series transition metal borides with low B content." On the other hand their crystals contain rather a large content of B: (Nb_2 B_3) and (Ta_2 B 3). Please clarify this aspect!
Response 2: For transition metal borides (TMBs), a straightforward way to obtain a high hardness was to incorporate more light atoms into the transition metal to form strong covalent bonds. Among them, transition-metal borides (TM1-xBx) including diborides of Os and Re, triborides (originally reported as tetraborides) of W and Mo, tetraborides of Cr, Mn and Fe, and dodecaborides of Zr, Y and Sc, have been the focus of promising superhard materials. Thus, the borides TMxBy (y:x<2:1) is called Low boron borides, See Ref.Y. Liang, P. Qin, H. Jiang, L. Zhang, J. Zhang, C. Tang, Designing superhard metals: The case of low borides, AIP Advances 8(2018) 045305.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The submitted manuscript contains the theoretical analysis of the effect of pressure and temperature on the physical properties of two superhard materials namely Nb2B3 and Ta2B3. The calculations were conducted correctly, the presented results are new and publishable. Some minor points should be clarified before final acceptance:
1) The following sentence in the abstract is too general:
"The atomic interaction of Nb2B3 and Ta2B3 is a combination of the metallic bond, covalent bond and some ionic bond."
Because there are differences between these compounds Authors should be more specific or simply remove this statement.
2) For clarity, the order of presented results should be slightly changed, i.e. the structural parameters and information about the space group should be presented at the beginning of the results section instead of Computational details.
3) Some more details should be given about methods and approximations. For example, in the following statement:
"The Debye temperature of Nb2B3 and Ta2B3 at 0 GPa and 0 K is 823.2 K and 605.9 K, in consistent with the results 833.4 K and 610.4 K based on the equation using the first principles [25]"
"The first principles" term is too general. A probably more accurate description will be "based on the calculation through the elastic constant method". The Niu model should be explained more clearly in the text. This approach has been introduced recently (J. Appl. Phys. 125(2019) 065105) and is still not a standardly used concept with which the general reader would be familiar.
4) In Figs. 4b and 4c there is Ta2B2 instead of Ta2B3.
Once these points will be clarified the paper should be accepted for publication.
Author Response
Response to Reviewer 2 Comments
Point 1: The following sentence in the abstract is too general:
"The atomic interaction of Nb2B3 and Ta2B3 is a combination of the metallic bond, covalent bond and some ionic
bond."
Because there are differences between these compounds Authors should be more specific or simply remove this
statement.
Response 1: "The atomic interaction of Nb2B3 and Ta2B3 is a combination of the metallic bond, covalent bond and
some ionic bond." in the abstract has been deleted. But we have modified as “The combination of relatively high
Vickers hardness and fracture toughness is determined by the metallic bond and covalent bond. With the increased
pressure, Nb2B3 possesses the stronger strength of B-B bonds than Ta2B3, which leads to the high hardness and Debye
temperature ”, see lines 17-20.
Point 2: For clarity, the order of presented results should be slightly changed, i.e. the structural parameters and
information about the space group should be presented at the beginning of the results section instead of Computational
details.
Response 2: The crystal structures, lattice constants and space group are presented at the beginning of the results
section, see lines 53-65.
Point 3: Some more details should be given about methods and approximations. For example, in the following
statement:
"The Debye temperature of Nb2B3 and Ta2B3 at 0 GPa and 0 K is 823.2 K and 605.9 K, in consistent with the results
833.4 K and 610.4 K based on the equation using the first principles [25]"
"The first principles" term is too general. A probably more accurate description will be "based on the calculation
through the elastic constant method". The Niu model should be explained more clearly in the text. This approach has
been introduced recently (J. Appl. Phys. 125(2019) 065105) and is still not a standardly used concept with which the
12
general reader would be familiar.
Response 3: Please see the attached file.
Point 4: In Figs. 4b and 4c there is Ta2B2 instead of Ta2B3.
Response 4: The Ta2B2 in Figure 2 has been modified as Ta2B3.
Author Response File: Author Response.pdf
