Crystal growth of La2/3-xLi3xTiO3 by the TSFZ method

Double-perovskite-type La2/3-xLi3xTiO3 (LLT) crystals were grown by the travelling solvent floating zone (TSFZ) method. When the floating zone (FZ) crystal growth method was applied, the La2Ti2O7 phase was deposited as an inclusion in the initial growth region. Using the TSFZ crystal growth method, however, inclusion-free LLT crystals were obtained for a 10 mol% La2Ti2O7-poor composition solvent relative to the stoichiometric LLT crystals. The molten zone was initially unstable as a result of habit plane formation during the crystal growth; however, the molten zone was stably maintained for a long period of time by decreasing the feed rate compared with the growth rate. Hence, LLT crystals of approximately 5 mmφ and 37 mm in length were obtained. The anisotropic ionic conductivity of the crystals annealed in air was σ[110]/σ[001] ≈ 3, with σ[110] = 1.64 × 10−3 S cm−1 and σ[001] = 5.26 × 10−4 S cm−1. LLT single crystals are candidates for high-performance solid-state electrolytes in all-solid-state Li ion batteries.

This paper describes crystal growth of LLT by TSFZ method. They paid a lot of attention to decomposition of the compound, incongruent melting, to achieve stable growth conditions. As a result, they succeeded to obtain really bulk single crystals. I think this is a very good work in terms of crystal growth.
To make this manuscript more interesting one for the readers, I encourage authors to add some results and descriptions.
Authors mentioned, 'decrease the growth temperature to prevent Li evaporation from the molten zone.' However, they did not disclose the effect of Li loss. It can be possible to estimate Li loss during growth by measuring total weight of feed rod and obtained crystals. I know not only loss of lithium but also oxygen deficiency is another cause of weight loss. However, rough estimation of Li loss may be possible.
I suppose Li-loss is also an important fact relating to the authors claim. In order to conclude incongruent melting, it is necessary to show that change of composition in molten zone was not due to loss of lithium but due to incongruent melting. To mention about phase diagram, chemical composition of the system must be kept constant during the process. Loss of lithium is a potential cause for misleading of phase relationships. I do not know if the authors have results to mention about Li loss during growth. However, I would like to ask the author to give quantitative or qualitative estimation of Li-loss during growth process.
The conductivity is affected by the number of carrier and mobility. It seems that anisotropy in conduction was more significant than the value reported in a literature. I suppose this is an evidence for high quality of the crystal grown by the authors. However, it is not possible to discuss conductivity in detail only from the results measured at room temperature. To give useful information to readers, it is a good idea to show temperature dependence of conductivity. Evaluation of the activation energy of conduction is necessary for understanding of the conduction mechanism. As the quality of the crystal seems to be sufficiently high to evaluate reliable physical parameters, I encourage the authors to work on temperature dependence measurements.

03-Oct-2018
Dear Professor Tanaka: Title: Crystal growth of La<sub>2/3-x</sub>Li<sub>3x</sub>TiO<sub>3</sub> by TSFZ method Manuscript ID: RSOS-181445 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The paper describe the growth of (La, Li)TiO3 using the TSFZ method. LLT is a potential battery material, and therefore I consider the present work timely and worth publishing. However, there are few important points that authors should consider including in the manuscript to make it more credible and useful: 1. Powder x-ray diffraction pattern of the successfully grown crystals (in Fig. 4) should be included to confirm their crystal structure (this is important since the structure is governed by the final Li concentration, and Li being volatile the final composition may differ from the starting composition). 2. EDX or EPMA compositions (average value and standard deviation) of the successfully grown crystals should be included. 3. It would be nice if AC conductivity of oxygen annealed crystals is also included. 4. Habit plane should be defined. It is not clear from the description given in paragraph 1 of section 3.3 as to how the habit plane formation hinders the growth. However, the paragraph 2 is clear where feeding speed is discussed. I will appreciate if paragraph 1 is also made more useful. 5. Figure 3 is not useful as it doesn't reveal much. You can however choose a picture showing the habit planes. If such a picture is not available you can remove Fig. 3, as you will anyway have one extra figure showing the powder XRD of the crystals. 6. No mention of Li evaporation during the growth? In my view, this is an important parameter for reproducibility of these experiments. I suggest that this point be mentioned and discussed.

Reviewer: 2
Comments to the Author(s) Dear authors, This paper describes crystal growth of LLT by TSFZ method. They paid a lot of attention to decomposition of the compound, incongruent melting, to achieve stable growth conditions. As a result, they succeeded to obtain really bulk single crystals. I think this is a very good work in terms of crystal growth.
To make this manuscript more interesting one for the readers, I encourage authors to add some results and descriptions.
Authors mentioned, 'decrease the growth temperature to prevent Li evaporation from the molten zone.' However, they did not disclose the effect of Li loss. It can be possible to estimate Li loss during growth by measuring total weight of feed rod and obtained crystals. I know not only loss of lithium but also oxygen deficiency is another cause of weight loss. However, rough estimation of Li loss may be possible.
I suppose Li-loss is also an important fact relating to the authors claim. In order to conclude incongruent melting, it is necessary to show that change of composition in molten zone was not due to loss of lithium but due to incongruent melting. To mention about phase diagram, chemical composition of the system must be kept constant during the process. Loss of lithium is a potential cause for misleading of phase relationships. I do not know if the authors have results to mention about Li loss during growth. However, I would like to ask the author to give quantitative or qualitative estimation of Li-loss during growth process.
The conductivity is affected by the number of carrier and mobility. It seems that anisotropy in conduction was more significant than the value reported in a literature. I suppose this is an evidence for high quality of the crystal grown by the authors. However, it is not possible to discuss conductivity in detail only from the results measured at room temperature. To give useful information to readers, it is a good idea to show temperature dependence of conductivity. Evaluation of the activation energy of conduction is necessary for understanding of the conduction mechanism. As the quality of the crystal seems to be sufficiently high to evaluate reliable physical parameters, I encourage the authors to work on temperature dependence measurements.

Appendix A
We thank referees for careful reading our manuscript and for giving useful comments.
We are looking forward to publishing our manuscript in Royal Society Open Science.
Our responses to the referee's comments are as follows: Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The paper describe the growth of (La, Li)TiO3 using the TSFZ method.
LLT is a potential battery material, and therefore I consider the present work timely and worth publishing. However, there are few important points that authors should consider including in the manuscript to make it more credible and useful: Fig. 4) should be included to confirm their crystal structure (this is important since the structure is governed by the final Li concentration, and Li being volatile the final composition may differ from the starting composition). →We thank the referee for fruitful suggestions. We included the XRD pattern of the grown crystal on Fig 4. We confirmed that the crystal structure of grown crystal is a tetragonal double-perovskite structure. We think that the final Li composition may differ from the starting composition. However, we obtained LLT crystal with a tetragonal doubleperovskite structure.

Powder x-ray diffraction pattern of the successfully grown crystals (in
We added following sentence in section 3.3: Figure 4 shows the XRD patterns of the LLT feed rod sintered at 1300°C for 12 h in air and the grown crystals. The XRD pattern of the grown crystals is attributed to a tetragonal double-perovskite-type LLT. Thus, we successfully obtained double-perovskite-type LLT crystals by TSFZ method. The diffraction peaks of the grown crystals shifted to lower angle than that of the feed rod. It had been reported that the LLT lattice expands as the extra Li evaporation [21]. The peak shift in the grown crystals agrees with the results of the quantitative analysis by EPMA. The diffraction intensity due to the superstructure in the grown crystals was high as compared with the sintered LLT. This indicates that the grown crystals have a high crystallinity.

EDX or EPMA compositions (average value and standard deviation) of the successfully grown crystals should be included.
→Thank you for your suggestion. We included the EPMA compositions (average and standard deviation).
We added the following sentences in section 3.3: The chemical composition of the grown crystals was determined to be La0.61±0.02Li0.17±0.006TiO3 (x=0.057±0.002) by quantitative analysis using EPMA. The Li content in the grown crystals was lower than that in the feed. The low Li content is due to evaporation of Li from the molten zone during the crystal growth and a lower distribution coefficient of Li into LLT.
We also added the experimental method of the quantitative analysis in Section "Experimental Procedure" as follows; Also the concentration of La and Ti in the grown crystals were determined by quantitative analysis using a La2Ti2O7 single crystal as a standard sample, and Li concentration 3x in the composition La2/3-xLi3xTiO3 was estimated using atomic ratio La/Ti.

It would be nice if AC conductivity of oxygen annealed crystals is
also included. → As described the first sentence in section 3.4, the crystals for the measurement were already annelid in oxygen. We added " The crystals were annealed in oxygen." also in figure caption of Fig. 5.

Habit plane should be defined. It is not clear from the description
given in paragraph 1 of section 3.3 as to how the habit plane formation hinders the growth. However, the paragraph 2 is clear where feeding speed is discussed. I will appreciate if paragraph 1 is also made more useful. →We deleted sentence in paragraph 1 of section 3.3 due to deletion of Fig. 3.

No mention of Li evaporation during the growth? In my view, this
is an important parameter for reproducibility of these experiments. I suggest that this point be mentioned and discussed. →We agree with you and have incorporated this suggestion. From the EPMA composition, Li composition in the grown crystals was lower than the starting composition. The low Li content in the grown crystals is duo to not only Li evaporation during growth also a low distribution coefficient of Li into LLT. In general, the melt growth causes a decrease of a solute into solid solution in the case of a distribution coefficient of a solute into a solid solution lower than unity. We added data by quantitative analysis, and mentioned the Li evaporation in section 3.3 as follows.
The chemical composition of the grown crystals was determined to be La0.61±0.02Li0.17±0.006TiO3 (x=0.057±0.002) by quantitative analysis using EPMA. The Li content in the grown crystals was lower than that in the feed. The low Li content is due to evaporation of Li from the molten zone during the crystal growth and a lower distribution coefficient of Li into LLT.

Reviewer: 2
Comments to the Author(s) Dear authors, This paper describes crystal growth of LLT by TSFZ method. They paid a lot of attention to decomposition of the compound, incongruent melting, to achieve stable growth conditions. As a result, they succeeded to obtain really bulk single crystals. I think this is a very good work in terms of crystal growth.
To make this manuscript more interesting one for the readers, I encourage authors to add some results and descriptions.
Authors mentioned, 'decrease the growth temperature to prevent Li evaporation from the molten zone.' However, they did not disclose the effect of Li loss. It can be possible to estimate Li loss during growth by measuring total weight of feed rod and obtained crystals. I know not only loss of lithium but also oxygen deficiency is another cause of weight loss. However, rough estimation of Li loss may be possible. → We thank the referee for fruitful suggestions. We observed Li vaporization during growth because a quartz tube around the growth region becomes white by deposition of Li compound into the quartz tube even though TSFZ growth.
We corrected "prevent" to "reduce" in the sentence which you suggested as follows: decrease the growth temperature to reduce Li evaporation from the molten zone.
We also added the results of the quantitative analysis by EPMA in section 3.3 and discussed about Li vaporization as follows: The chemical composition of the grown crystals was determined to be La0.61±0.02Li0.17±0.006TiO3 (x=0.057±0.002) by quantitative analysis using EPMA. The Li content in the grown crystals was lower than that in the feed. The low Li content is due to evaporation of Li from the molten zone during the crystal growth and a lower distribution coefficient of Li into LLT.
I suppose Li-loss is also an important fact relating to the authors claim.
In order to conclude incongruent melting, it is necessary to show that change of composition in molten zone was not due to loss of lithium but due to incongruent melting. To mention about phase diagram, chemical composition of the system must be kept constant during the process. Loss of lithium is a potential cause for misleading of phase relationships. I do not know if the authors have results to mention about Li loss during growth. However, I would like to ask the author to give quantitative or qualitative estimation of Li-loss during growth process. →Thank you for your suggestion. We measured chemical compositions of the grown crystals by EPMA. We think that Li evaporate during the crystal growth because Li composition in grown crystals differs from the starting composition. We mentioned the chemical compositions of the grown and Li evaporation in section 3.3.
The conductivity is affected by the number of carrier and mobility. It seems that anisotropy in conduction was more significant than the value reported in a literature. I suppose this is an evidence for high quality of the crystal grown by the authors. However, it is not possible to discuss conductivity in detail only from the results measured at room temperature. To give useful information to readers, it is a good idea to show temperature dependence of conductivity. Evaluation of the activation energy of conduction is necessary for understanding of the conduction mechanism. As the quality of the crystal seems to be sufficiently high to evaluate reliable physical parameters, I encourage the authors to work on temperature dependence measurements.
→We thank you for fruitful suggestions. Although we think that it is a good idea to show temperature dependence of conductivity to give useful information, we can not measure a temperature dependence of conductivity because there is no equipment attached heater. However, we work toward to evaluate the activation energy of conduction in the future work.