Crystal Structure and Electrical Properties of Ruthenium-Substituted Calcium Copper Titanate

Round 1

Reviewer 1 Report

This work reports a study of crystal structure and electric properties of ruthenium-substituted calcium-copper titanates. The authors performed an extensive investigation and shown several experimental data. The manuscript can be published after some revisions. In particular, writing style should be improved as well as grammar and syntax. In addition, several typos are present throughout the text. Please, find in what follow other specific comments.

1.     Page 3, line 112 and Page 7, line 211: Why is here mentioned only the CCTRO system?

2.   Page 5, lines 171-177: This part is redundant. Figure 2 already shows this information.

3.       Page 6, lines 187 and 192: It is not clear the meaning of “single-crystalline”.

4.     Figure 4: I suggest checking the reported interlayer distance. These values appear inverted.

5.     Table 4: Data are reported twice.

6.     Figure 8: Value of sigma0 should be also reported.

7.    Figure 8. Reported values of sigma do not match with Table 4.

8.     Figure 9: the y-axis should be rescaled, i.e., epsilon_prime up to 2x10⁴ for instance.

9.     Page 10, lines 280-283. Statement and conclusion are not supported by the data reported in Table 5.

--   Figure 10: It is not specified in the text what material this figure is related to.

Author Response

We are thankful for positive evaluation and a recommendation from the Reviewer 1. According to the suggestions of Reviewer 1 we made all necessary corrections.

This work reports a study of crystal structure and electric properties of ruthenium-substituted calcium-copper titanates. The authors performed an extensive investigation and shown several experimental data. The manuscript can be published after some revisions. In particular, writing style should be improved as well as grammar and syntax. In addition, several typos are present throughout the text. Please, find in what follow other specific comments.

Point 1:. Page 3, line 112 and Page 7, line 211: Why is here mentioned only the CCTRO system?

Response 1: CCTRO is the abbreviation for CaCu₃Ti_4-xRu_xO₁₂, thus includes all examined powders, while CCTO, CCT3RO and CCRO are used for certain compounds where x = 0, 1 and 4, respectively. Beside on Page 3, line 113 and Page 7, line 211, the abbreviation CCTRO is used many times in the manuscript.

Point 2: Page 5, lines 171-177: This part is redundant. Figure 2 already shows this information.

Response 2: According to the Reviewer 1 suggestion text from the Page 5, lines 171-177 is removed from the revised manuscript.

Point 3: Page 6, lines 187 and 192: It is not clear the meaning of “single-crystalline”.

Response 3: We used term single-crystalline as a synonym for term monocrystalline. Actually, the idea was to emphasize agglomeration of small monocrystalline particles (inset in Figures 3a and 3c).

Point 4: Figure 4: I suggest checking the reported interlayer distance. These values appear inverted.

Response 4: The interlayer distances reported in Fig 4c were checked and by omission inverted values were reported. Figure 4 with corrected interlayer distances is provided in the revised manuscript.

Point 5: Table 4: Data are reported twice.

Response 5: We are thankful for this observation. In the revised manuscript Table 4 is corrected.

Point 6: Figure 8: Value of sigma0 should be also reported.

Response 6: In the revised manuscript value of sigma0 is denoted in Figure 8.

Point 7: Figure 8. Reported values of sigma do not match with Table 4.

Response 7: The authors made an omission. Figure 8 which match with the values presented in Table 4 is included in the revised manuscript.

 

Point 8: Figure 9: the y-axis should be rescaled, i.e., epsilon_prime up to 2x10⁴ for instance.

Response 8: Figure 9 is revised according to the Reviewer 1 suggestion; the y-axis is rescaled to 2x10⁴.

 

Point 9: Page 10, lines 280-283. Statement and conclusion are not supported by the data reported in Table 5.

Response 9: The statement from Page 10, lines 280-283 is excluded from the revised manuscript.

 

Point 10: Figure 10: It is not specified in the text what material this figure is related to.

Response 10: The authors are thankful to the Reviewer 1 for suggestions. The figure 10 shows the influence of temperature and frequency on the dielectric loss (tgδ) for CCTO ceramic, which is additionally stated in the text and in the figure caption.

Author Response File: Author Response.pdf

Reviewer 2 Report

The results are of potential interest to readers in related areas. The manuscript is recommended for publication after addressing the following issues.

(a) The manuscript should be polished in language.

(b) Electronic structructures of elements/ions could be removed.

(c) Equations/formulas should be numbered.

(d) Some figures should be enlarged.

Author Response

We are thankful for a detailed analysis of our results and constructive suggestions which encourage us to improve the manuscript.

 

The results are of potential interest to readers in related areas. The manuscript is recommended for publication after addressing the following issues.

 

Point 1: The manuscript should be polished in language.

Response 1: English language is edited by a native English speaker. All corrections are yellow highlighted in the revised manuscript.

 

Point 2: Electronic structures of elements/ions could be removed.

Response 2: The authors believe that the electronic structures of elements/ions could be useful for easier understanding of differences in electrical conductivity, so prefer to keep them.

 

Point 3: Equations/formulas should be numbered.

Response 3: According to the Reviewer 2 suggestion, equations/formulas are numbered in the revised manuscript.

 

Point 4: Some figures should be enlarged.

Response 4: We have uploaded all figures in acceptable format (TIFF) and with appropriate resolution. We believe that technical editing will be done by the Journal management after accepting the manuscript.

 

 

Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors reported the electrical and dielectric properties of CCTO doped with Ru in the TiO2 sites. The novelty of this work is clear because, to the best of my knowledge, there is no report on the dielectric properties of this material (even though the dielectric properties of CCTO/RuO2 have ever been reported). However, there are many points that the authors must carefully consider revising clearly before consideration for publication in Materials.

 

In the Abstract, the important numerical results must be included in the Abstract such as the dielectric constant at 1 RT and 1 kHz, loss tangent, etc.

 

In the Introduction, the motivation of this work was not clear. The literature review on the relevant previous works must be re-written. The replacement of Ti4+ with isovalent ions should be mentioned. The dielectric properties of isovalent dopant like Ru4+ must be mentioned and reviewed in the Introduction such as Sn4+ (https://doi.org/10.1016/j.jeurceramsoc.2021.04.017), Zr4+ (https://doi.org/10.1016/j.jallcom.2013.01.090;),Ge4+ (https://doi.org/10.1016/j.jallcom.2021.160322), etc.

 

As well known, the dielectric properties of CCTO ceramics are related to their microstructure. Thus, more details of microstructure analyses should be represented such as the SEM images of the sintered ceramics, the mean grain size, relative densities of the sintered ceramics.

 

Fig. 9, y-scale must be optimized to clearly represent the overall dielectric constant. The maximum scale may be 20k.

 

Table 5, the authors showed the dielectric constant, but not include the dielectric loss tangent.

 

The possible mechanism of a large value of conductivity / loss tangent must be explained. This result was likely related to the percolation effect like in the previous work for CCTO/RuO2 composites (http://dx.doi.org/10.1063/1.4893009)?

 

 

Author Response

We are very thankful to the reviewers for this thorough review. We have revised our manuscript according to the Reviewer's useful suggestions and comments.

 

In this work, the authors reported the electrical and dielectric properties of CCTO doped with Ru in the TiO2 sites. The novelty of this work is clear because, to the best of my knowledge, there is no report on the dielectric properties of this material (even though the dielectric properties of CCTO/RuO₂ have ever been reported). However, there are many points that the authors must carefully consider revising clearly before consideration for publication in Materials.

 

Point 1: In the Abstract, the important numerical results must be included in the Abstract such as the dielectric constant at 1 RT and 1 kHz, loss tangent, etc.

Response 1: We agree with the Reviewer 3 that some important numerical results missing in the Abstract. However, allowed number of words in the Abstract is 200 which we already reached. To include new data, it is necessary to completely rewrite the Abstract and exclude some of presented ones. If all three Reviewers and the Editor consider it to be necessary, we will do it. 

Point 2: In the Introduction, the motivation of this work was not clear. The literature review on the relevant previous works must be re-written. The replacement of Ti⁴⁺ with isovalent ions should be mentioned. The dielectric properties of isovalent dopant like Ru⁴⁺ must be mentioned and reviewed in the Introduction such as Sn⁴⁺ (https://doi.org/10.1016/j.jeurceramsoc.2021.04.017), Zr⁴⁺ (https://doi.org/10.1016/j.jallcom.2013.01.090;), Ge⁴⁺ (https://doi.org/10.1016/j.jallcom.2021.160322), etc.

Response 2: In the revised manuscript the Introduction is rewritten and the replacement of Ti⁴⁺ with isovalent cations such as Sn⁴⁺, Zr⁴⁺, Ge⁴⁺ is emphasized while three new references are added in the Introduction and in the Reference list.

Added references:

1. Li, M.; Cai, G.; Zhang, D.F.; Wang, W.Y.; Wang, W.J.; Chen, X.L. Enhanced dielectric responses in Mg-doped CaCu₃Ti₄O₁₂, J. Appl. Phys. 2008, 104, 074107. doi: 10.1063/1.2989124
2. Boonlakhorn, J.; Chanlek, N.; Srepusharawoot, P.; Thongbai, P. Improved dielectric properties of CaCu_3-xSn_xTi₄O₁₂ ceramics with high permittivity and reduced loss tangent, J. Mater. Sci. Mater. Electron. 2020, 31, 15599–15607. doi: 10.1007/s10854-020-04123-x
3. Boonlakhorn, J.; Thongbai, P.; Putasaeng, B.; Kidkhunthod, P.; Maensiri, S.; Chindaprasirt, P. Microstructural evolution, non-Ohmic properties, and giant dielectric response in CaCu₃Ti_4-xGe_xO₁₂ ceramics, J. Am. Ceram. Soc. 2017, 100, 3478–3487. doi: 10.1111/jace.14886
4. Xu, D.; Zhu, Y.; Zhang, B.; Yue, X.; Jiao, L.; Song, J.; Zhong, S.; Ma, J.; Bao, L.; Zhang, L. Excellent dielectric performance and nonlinear electrical behaviors of Zr-doped CaCu₃Ti₄O₁₂ thin films, J. Mater. Sci.: Mater. Electron. 2018, 29, 5116–5123. doi:10.1007/s10854-017-8475-0
5. Xu, Z.; Qiang, H. Enhanced dielectric properties of Zn and Mn co-doped CaCu₃Ti₄O₁₂ ceramics, J. Mater. Sci. Mater. Electron. 2017, 28, 376–380. doi: 10.1007/s10854-016-5533-y
6. Xu, Z.; Qiang, H.; Chen, Y.; Chen, Z. Microstructure and enhanced dielectric properties of yttrium and zirconium co-doped CaCu₃Ti₄O₁₂ ceramics, Mater. Chem. Phys. 2017, 191, 1–5. doi: 10.1016/j.matchemphys.2017.01.015
7. Boonlakhorn, J.; Chanlek, N.; Manyam, J.; Srepusharawoot, P.; Thongbai, P. Simultaneous two-step enhanced permittivity and reduced loss tangent in Mg/Ge-Doped CaCu₃Ti₄O₁₂ ceramics, J. Alloys Compd. 2021, 877, 160322. doi: 10.1016/j.jallcom.2021.160322

 

In the revised manuscript number of the references in the Reference list is 38 instead 31.

 

The aim of our work was to optimize the amount of ruthenium in the CaCu₃Ti_4-xRu_xO₁₂ crystal structure in order to obtain inexpensive and commercially acceptable materials with the desired functional properties – i.e. to synthesize compound isostructural with CaCu₃Ti₄O₁₂ but with much larger conductivity which can be successfully used as interlayer between dielectric ceramic and metallic electrode with the aim to reduce interlayer stress.

To be more precise, one sentence is added in the revised manuscript “Quite contrary, our interest was to synthesize CaCu₃Ti_4-xRu_xO₁₂ as compound isostructural with CaCu₃Ti₄O₁₂ but with much larger conductivity which can be successfully used as interlayer between dielectric ceramic and metallic electrode with the aim to reduce interlayer stress.”

 

Point 3: As well known, the dielectric properties of CCTO ceramics are related to their microstructure. Thus, more details of microstructure analyses should be represented such as the SEM images of the sintered ceramics, the mean grain size, relative densities of the sintered ceramics.

Response 3: We agree that microstructure influences dielectric properties of material. The results related to the effect of sintering to CCTO ceramics microstructure and dielectric properties we published earlier in our paper: Marković, S., Lukić, M., Jovalekić, Č., Škapin, S.D., Suvorov, D., Uskoković, D. Sintering Effect on Microstructure and Electrical Properties of CaCu₃Ti₄O₁₂ ceramics. Ceram. Trans. 2013, 240, 337–348, doi: 10.1002/9781118744109.ch37.

In our future research we are planning to examine an influence of different sintering conditions (different heating rate, final temperature, dwell time and sintering atmosphere – air and argon) on dielectric properties of CaCu₃Ti₄O₁₂, CaCu₃Ti₃RuO₁₂, and CaCu₃Ti₄O₁₂/CaCu₃Ti₃RuO₁₂ functionally graded materials. That is why in this manuscript microstructure of sintered samples was not included.

As we stated in the Introduction the primary goal of this study is to show that obtaining of commercially acceptable materials with the desired functional properties is possible by substation of one Ru ion in CCTO:

For the fabrication of capacitors with the good capacitive performance, it is important to combine dielectric ceramic and electrodes with similar crystal structures and unit cell parameters. The reduction of the stress on the ceramic-electrode interfaces can be achieved by using commercially available materials as an interlayer having a close lattice parameter match with both dielectric and electrode. It was noticed that the incorporation of Ru⁴⁺ ions in the CaCu₃Ti₄O₁₂ crystal structure dramatically increases the conductivity of these materials. The CaCu₃Ru₄O₁₂ materials are isostructural with CaCu₃Ti₄O₁₂ materials with cubic space group showing the Pauli-paramagnetic and metallic character. With the CaCu₃Ru₄O₁₂ material as an interface between CaCu₃Ti₄O₁₂ ceramic and metallic electrode, it is possible to reduce the stress on the dielectric-electrode interfaces.

 

Since ruthenium in an expensive chemical element, the aim of our work was to optimize the amount of ruthenium in the CaCu₃Ti_4-xRu_xO₁₂ crystal structure in order to obtain inexpensive and commercially acceptable materials with the desired functional properties – i.e. to synthesize compound isostructural with CaCu₃Ti₄O₁₂ but with much larger conductivity.

 

Point 4: Fig. 9, y-scale must be optimized to clearly represent the overall dielectric constant. The maximum scale may be 20k.

Revised 4: According to shared suggestion of the Reviewers 1 and 3 Figure 9 is revised; the y-axis is rescaled to 2x10⁴.

 

Point 5: Table 5, the authors showed the dielectric constant, but not include the dielectric loss tangent.

Response 5: Table 5 shown in the revised manuscript contains both the dielectric constant and the dielectric loss tangent values.

 

Point 6: The possible mechanism of a large value of conductivity / loss tangent must be explained. This result was likely related to the percolation effect like in the previous work for CCTO/RuO₂ composites (http://dx.doi.org/10.1063/1.4893009)?

Response 6: In the paper “Enhancement of high dielectric permittivity in CaCu₃Ti₄O₁₂/RuO₂ composites in the vicinity of the percolation threshold”, by Rupam Mukherjee, Gavin Lawes, and Boris Nadgorny, published in APPLIED PHYSICS LETTERS 105, 072901 (2014), the authors examined composite nanoparticle system consisting of metallic RuO₂ grains embedded into CaCu₃Ti₄O₁₂ matrix while we examined powders which are a single-phase perovskite oxide without a secondary phase.

The aim of Mukherjee, Lawes and Nadgorny, was to increase the dielectric constant of CCTO by introduction of RuO₂, and production of RuO₂/CCTO composites. Quite contrary, our aim was to enhance conductivity.

 

As we stated in the manuscript, different conductivity between the CCTO and CCT3RO samples can be explained by the Kondo mechanism:

The difference in electrical conductivity can be explained by different electron configurations of Ti⁴⁺ and Ru⁴⁺ ions. Titanium is a 3d element with an electronic configuration:

Ti: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d²

However, the electronic configuration of Ti⁴⁺ cation in the CCTO crystal structure turns into:

Ti⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶

with closed external 3p⁶ shells which explain the relatively low conductivity of these materials.

The isomorphic replacement of the Ti⁴⁺ ions by Ru⁴⁺ leads to different electronic configurations. Explicitly, Ru⁴⁺ has the electronic configuration:

Ru⁴⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s⁰ 4d⁴,

with uncompleted d subshells. These four electrons in the octahedral crystalline environment are situated in tg crystalline orbitals containing two unpaired d electrons, i.e., Ru⁴⁺ represents a magnetic ion in the CCRO crystal. The CCRO conductivity can be explained by the Kondo mechanism. Namely, CCRO materials are d-electron heavy-fermion system, which behaves similarly to an f-electron heavy-fermion system. In CCRO structures the Cu 3d electrons are localized, while 4d electrons of magnetic Ru⁴⁺ ions contribute to the conductivity.

The authors remark:

The authors noticed omission in the Arrhenius equation, page 9; the omission is corrected in the revised manuscript. Actually,  lnσ= σ₀exp⟨-E_a/k_BT⟩  is corrected to be σ=σ₀exp⟨-Ea/k_BT⟩  

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revised manuscript can be now accepted.

Reviewer 2 Report

The manuscript has been well revised. It is recommended for publication as it is.

Reviewer 3 Report

All comments have been revised. Accepted.