Hydrogen-Induced Order–Disorder Effects in FePd3
Round 1
Reviewer 1 Report
The authors have performed an extensive in-situ and ex-situ X-ray and neutron diffraction investigation in the low solid solubility region of hydrogen and deuterium in Pd3Fe. Their results complete and confirm previous reports on this subject. The “hydrogenation terminology” used throughout the manuscript is not always the correct one, or at least not the accepted one in hydrogen absorption research.
Several comments follow:
A. FePd3H0.047(9) (row 16 in the abstract) is not a hydride but an intermetallic compound with a small amount of dissolved hydrogen in it, i.e. a solid solution region of hydrogen in metal. The term hydride is used when there exists a metal to hydride phase transition. Most intermetallic compounds and some elementary metals transform to hydrides by first order phase transition below some critical temperature, Tc. For example, there is a metal to hydride phase transition in the Pd-H2 system below a critical temperature of about 565 K. It is possible to reach the hydrogen-rich side in a smooth way, i.e. without going through first order phase transition, at temperatures above Tc. Such hydrogen-rich compounds may still be termed “hydrides”, because at some lower temperatures they undergo the mentioned first order metal to hydride phase transition. In no way, FePd3H0.047(9) falls into this classification. The authors are advised to omit “hydride(s)” and “deuteride(s)” throughout their work for the small dissolved hydrogen and deuterium contents in FePd3. In addition, in the Keywords the authors are advised to write “dissolved hydrogen” instead of “metal hydrides” and “interstitial hydrogen”, “interstitial deuterium” instead of “interstitial hydrides, deuterides”.
B. Fig. 6 and part of rows 318-343: As minor remarks for this comment, it is suggested to write after “blue symbols from this work” – row 327, “see left side of Fig. 4b”. Also, in the upper part of Fig. 6 there are 4 points from the literature, but in the lower part there are five. Why? The more significant remarks follow: 1. It is suggested, for the sake of consistency, to keep the hydrogen content in the upper and lower part (in my copy it is not left and right) of Fig. 6 as hydrogen atoms per formula unit (H/FePd3). 2. It is not clear whether the p0.5 scale (square root of the pressure) in the lower part is correct. For example, the square root of the highest pressure (8 MPa or 8,000 kPa), applied in this work, is 89.4 kPa0.5. The square roots of the literature points of about 40 MPa (40,000 kPa) and 100 Mpa (100,000 KPa) in the upper part of Fig. 6 are 200 and 316.2 kPa0.5, respectively. Such values of p0.5 do not appear in the lower part of Fig.6. 3. It is stated in lines 340-343 that the deviation from Sieverts’ law have kinetic meaning (“the rate limiting step of hydride formation”). The points on the Sieverts’ plot are equilibrium points and have no connection whatsoever to kinetics! The deviation from Sieverts’ law indicates that the dissolved hydrogen atoms do not constitute anymore an ideal solid solution, i.e. the hydrogen atoms interact with each other. This occurs at some point of higher hydrogen concentrations and higher hydrogen pressures. 4. Isn’t it necessary to consider fugacity for the high-pressure points in Fig. 6. If yes, this neglection should at least be pointed out.
The rest of the comments are of technical character:
- Row 14, row 339 and elsewhere: It should be written Sieverts’ and not Sievert’s. It is written correctly in some other places. The name of the person is Sieverts and not Sievert. To correct where necessary.
- Rows 20-21: In the abstract it is mentioned that the order-disorder transition is a first order transition. This is not mentioned in the main text. Have I missed it?
- Rows 106-114: The authors indicate the wavelength of Co-Kα radiation. For the sake of completeness, the wavelength of Mo-Kα radiation may be also indicated.
- Rows 233-241: It is not clear why FePd3D0.03(1) was initially assumed (row234). In any way, volume 3.15 Å3 and 2.15 Å3 per interstitial deuterium atom may be estimated from the measured lattice constants if D0.03 or D0.044, respectively. Both values fall into the usual range of interstitial hydrogen volumes and thus indicate the precision of the measurements in this work.
Comments A and B should be particularly considered before proceeding further with the publication procedure.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The work reports a study of ordering effect in FePd3 on the intercalation of H atoms in the octahedral interstitial sites. The analysis of H insertion under the well designed hydrogen pressures over disordered and ordered FePd3 is systematical and well presented, which sheds light on the catalyst design of related materials. The work can be accepted for publication on Crystals.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The article is devoted to the study of the properties of binary intermetallic compounds based on iron-palladium compounds, the interest in which is due to a wide range of their practical applications, as well as the study of the fundamental foundations of their properties. In general, the presented direction of research is quite promising, the results obtained are not in doubt, as they are confirmed by a sufficiently large number of different research methods. In general, the article corresponds to the subject of the declared journal and can be accepted for publication after the authors answer a number of questions that the reviewer has when reading it.
1. In the introduction, the authors should describe in more detail the novelty and relevance of their work, as well as provide more comparative data from other works, reflecting the effectiveness of using the proposed compounds for generating hydrogen.
2. The presented diffraction patterns require a significant improvement in the quality of the survey, the authors should present the spectra in a better quality, and also reflect the change in the shape of the main lines, since it is impossible to assess the degree of deformation of X-ray reflections in the presented form.
3. The presented dependences of the structural parameters depending on various factors require additional explanations related to the description of the effects of broadening and contraction of the crystal lattice.
4. The data presented in Figure 6 require additional explanations, including what is the reason for the saturation with increasing pressure, and why such a spread between pressure values from 40 MPa to 100 MPa? Is this due to the lack of experimental data or other factors?
5. Regarding the additional material attached to the article, in this form there is too much of it, the authors should restructure the data presented and revise their presentation.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The authors have completely corrected all the comments indicated by the reviewer, the article can be accepted for publication.
