The Conversion Polymorphism of Perovskite Phases in the BiCrO₃–BiFeO₃ System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports on preparation and characterisation of crystal structure and magnetic properties of Cr-rich solid solutions in the BiCrO₃-BiFeO₃ system. Thousands of works are devoted to chemical modifications of bismuth ferrite aiming to adjust and control polar and magnetic orderings; the absolute majority of them are the cases of Bi-site substitutions. Formation of bulk solid solutions with substitutions in Fe-site using the conventional synthesis techniques is typically limited by few mol%. High-pressure (HP) synthesis is a way to extend solubility in BiFeO₃-based systems, where Fe³⁺ is substituted by another cation or a combination of cations with the net charge of 3+. The HP technique is a powerful tool but not widely available. Therefore, each new report on continuous solid solutions of a BiFeO₃-BiM³⁺O₃ type is particularly important.

The manuscript certainly deserves publication in MDPI Inorganics. I suggest some minor corrections before the manuscript is accepted.

(1) The Author uses abbreviation “HP” and “AP” for the phases as-prepared using HP synthesis and HP-prepared followed by annealing at ambient pressure, respectively. The same Author in his paper published in Chem. Mater. 2011, 23, 4505 (doi.org/10.1021/cm201774y) used “AP” to denote phases initially prepared at ambient pressure. It can bring misunderstanding. I suggest to introduce new abbreviation for the case of HP-prepared phases, which were then annealed: something like “annealed HP” or “converted HP”.

(2) To present the temperature dependence of the unit cell volumes of the involved crystalline phases an inset in Figure 6 or as a new figure would be useful.

(3) For better illustration, the experimental data from Table 2, in particular “T_N versus x” for each crystalline phase should be presented in a new figure.

Author Response

We thank the reviewers for their positive evaluation of our work and constructive suggestions to improve our paper. We addressed the main comments, and our detailed responses are given below.

 

Reviewer 1.

1. The Author uses abbreviation “HP” and “AP” for the phases as-prepared using HP synthesis and HP-prepared followed by annealing at ambient pressure, respectively. The same Author in his paper published in Chem. Mater. 2011, 23, 4505 (doi.org/10.1021/cm201774y) used “AP” to denote phases initially prepared at ambient pressure. It can bring misunderstanding. I suggest to introduce new abbreviation for the case of HP-prepared phases, which were then annealed: something like “annealed HP” or “converted HP”.

 

Our reply.

We thank the reviewer for this suggestion and basically agree with it. We indeed used the “AP” abbreviation in our old work, Chem. Mater. 2011, 23, 4505, for samples prepared directly at ambient pressure. However, we used the “AP” abbreviation in our recent paper, Inorganics 2024, 12, 226, for the converted HP phases. Therefore, we would like to keep this recent notation. Even though the notations, “annealed HP” or “converted HP”, are more correct, they are a little bit long to use often throughout the paper. The abstract and the experimental part clearly describe how the “AP” phases were prepared (through the conversion-polymorphism step), therefore, there should be no misunderstanding.

 

2. To present the temperature dependence of the unit cell volumes of the involved crystalline phases an inset in Figure 6 or as a new figure would be useful.

 

Our reply.

We thank the reviewer for this suggestion and agree that temperature dependence of the unit cell volumes also provides important information. Therefore, we added the part (b) to the revised Figure 6 with temperature dependence of the unit cell volumes.

 

3. For better illustration, the experimental data from Table 2, in particular “TN versus x” for each crystalline phase should be presented in a new figure.

 

Our reply.

We thank the reviewer for this suggestion and agree that such a figure will give a better illustration of the results. Therefore, we added a new figure 15 with a composition-temperature phase diagram (coupled with the request of reviewer 3 to show temperatures of magnetic and structural phase transitions for the end members of BiCrO3 and BiFeO3) into the revised version.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript investigate the structral phase relation of a very hot class of perovskit materials. The manuscript is well written and structured. The manuscript could be published as it is.

 

Author Response

We thank the reviewer for positive evaluation of our work. Reviewer 2 had no comments to address. 

Reviewer 3 Report

Comments and Suggestions for Authors

This study examines samples from the Cr-rich region of bulk BiCr_1−xFe_xO₃ solid solutions with 0.1 ≤ x ≤ 0.4. The authors have observed structural phase transitions to the Pnma modification accompanied by consistent variations in the phase transition temperature. The as-synthesized samples did not revert to their original state upon cooling from elevated temperatures, so demonstrating the occurrence of conversion polymorphism. Subtle incommensurate structural modulations were observed in BiCr_1−xFe_xO₃ solid solutions with 0.1 ≤ x ≤ 0.4. Comprehensive magnetic properties were documented as well.

This study is of high quality, with its aim within the scope of Inorganics. The results are well documented, discussed and presented. However, the author(s) should revise a few aspects, presented below.

Introduction is informative, yet could be supported by a nice figure or two, to increase the clarity. I.e., in lines 40-43 and 53-54, this aspect should be illustrated in a form of two figures, i.e. showing the (p,T) phase diagrams of those salts

Lines 301-309, how exactly the authors have determined the degree of substitution? Have you used any method like ICP-MS, XRF or ASA for elemental analysis?

While this is not mandatory for the current article, I really would like the author to consider using the periodic DFT calculations for their studies. This work is an excellent example of the project that could really benefit from such addition of the theoretical calculations, i.e. using CASTEP. Personally, I would start with substituting the Cr atoms by Fe and checking how the choice of the site affect the structural, electronic, magnetic and thermodynamic properties. I’m aware that in the case of non-stoichiometric substitution there are multiple options, but it can be handled. Those calculations could then explain, at the atomistic level, the experimentally observed results.

Figures 1 and 2, I really appreciate the high quality of the presented patterns. However, I would love to see the theoretical PXRD patterns, simulated based on the substituted (with mixed occupancies) CIFs.

Table 1, again, this is not a drawback, but I would like the Authors to explain why they have stopped the doping at the 0.4 level? Would the structures with higher Fe content be unstable? Of course, I don’t demand to study the whole range (0.0-1.0), but is should be commented.

Table 1, how the amount of impurities have been determined? Using QPA of the PXRD patterns? This should be described.

Figure 3, the peaks should be integrated, presented the values of enthalpies of those transitions. Those values of ΔH should be discussed as well.

Author Response

We thank the reviewers for their positive evaluation of our work and constructive suggestions to improve our paper. We addressed the main comments, and our detailed responses are given below.

 

Reviewer 3.

1. Introduction is informative, yet could be supported by a nice figure or two, to increase the clarity. I.e., in lines 40-43 and 53-54, this aspect should be illustrated in a form of two figures, i.e. showing the (p,T) phase diagrams of those salts

 

Our reply.

We thank the reviewer for this suggestion. We added a new figure 15 with a composition-temperature phase diagram into the discussion part as such a figure shows our results of the present work and some literature data including magnetic and structural phase transitions for the end members of BiCrO3 and BiFeO3 (as suggested by the reviewer).

 

2. Lines 301-309, how exactly the authors have determined the degree of substitution? Have you used any method like ICP-MS, XRF or ASA for elemental analysis?

 

Our reply.

We did not use any chemical methods to support the chemical compositions. The degree of substitution is clear from the starting chemical mixtures (stoichiometries) and the formation of nearly single-phase samples. In addition, Figure 5 with the compositional dependence of the lattice parameters gives support for the degree of substitution. It was emphasized as “Nearly linear increases of all parameters were observed with the increase of the Fe content in agreement with the larger ionic radius of Fe³⁺ cations (r_VI = 0.645 Å) in comparison with Cr³⁺ cations (r_VI = 0.615 Å) [67] confirming the formation of the solid solutions.”.

 

3. While this is not mandatory for the current article, I really would like the author to consider using the periodic DFT calculations for their studies. This work is an excellent example of the project that could really benefit from such addition of the theoretical calculations, i.e. using CASTEP. Personally, I would start with substituting the Cr atoms by Fe and checking how the choice of the site affect the structural, electronic, magnetic and thermodynamic properties. I’m aware that in the case of non-stoichiometric substitution there are multiple options, but it can be handled. Those calculations could then explain, at the atomistic level, the experimentally observed results.

 

Our reply.

We thank the reviewer for this suggestion. Our paper is a purely experimental work, and it has a lot of experimental data for one publication. While the theoretical understanding is important, it is out of the scope of the present paper. Such work could make a whole new paper. In addition, such calculations will take a considerable amount of time, much more than provided for the revision.

 

4. Figures 1 and 2, I really appreciate the high quality of the presented patterns. However, I would love to see the theoretical PXRD patterns, simulated based on the substituted (with mixed occupancies) CIFs.

 

Our reply.

We added such theoretical PXRD patterns into a supporting information file.

 

5. Table 1, again, this is not a drawback, but I would like the Authors to explain why they have stopped the doping at the 0.4 level? Would the structures with higher Fe content be unstable? Of course, I don’t demand to study the whole range (0.0-1.0), but is should be commented.

 

Our reply.

In the revised version, we provided the following explanation why only the x = 0.1-0.4 compositions were studied in this work: “We note that at x ³ 0.5, a different modification with space group R3c is formed [62] (that is confirmed by us); therefore, compositions with x ³ 0.5 were not included into the present work.”

 

6. Table 1, how the amount of impurities have been determined? Using QPA of the PXRD patterns? This should be described.

 

Our reply.

In the revised version of the experimental part, we provided the following details: “The reported weight fractions of all the phases were calculated by the RIETAN-2000 program [79] from the refined scale factors.”

 

7. Figure 3, the peaks should be integrated, presented the values of enthalpies of those transitions. Those values of ΔH should be discussed as well.

 

Our reply.

We thank the reviewer for this suggestion, and we agree that this is important information. Therefore, values of ΔH are reported in the revised Figure 3 and discussed in the text.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The Authors have revised and improved their work. Current version can surely be accepted.