Large Size Single Crystal Growth of Ti4O7 by the Floating-Zone

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Large Size Single Crystal Growth of TiO by the Floating-Zone Method Qing-Yuan Liu, Zi-Yi Liu, Xue-Bo Zhou, Zhi-Guo Liu, Ming-Xue Huo, Xian-Jie Wang, and Yu Sui Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01318 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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Crystal Growth & Design

Large Size Single Crystal Growth of Ti4O7 by the Floating-Zone Method Qing-Yuan Liu,† Zi-Yi Liu,† Xue-Bo Zhou,† Zhi-Guo Liu,† Ming-Xue Huo,*,‡ Xian-Jie Wang,*,† and Yu Sui*,† †Department

of Physics, Harbin Institute of Technology, Harbin 150001, China ‡Research Center of Basic Space Science, Harbin Institute of Technology, Harbin 150001, China

ABSTRACT: We report the growth of large-size Ti4O7 single crystals by the floating-zone method using an infrared image furnace. By employing a high rotation rate of the seed rod, we improved the temperature gradient at the solid–liquid interface, thereby suppressing the decomposition of Ti4O7 at high temperature. By systematically studying the influence of growth parameters on crystal quality, we found that pure argon atmosphere is a prerequisite for getting a pure Ti4O7 phase and that a moderate rotation rate of the seed rod is a critical element in obtaining high crystal quality. The X-ray rocking scan confirmed the quality of the grown single crystal with dimensions of 2.7 mm × 2.3 mm × 1.7 mm, which was further characterized by electrical resistivity measurements. With decreasing temperature, the electric resistances of the Ti4O7 single crystal along a, b, and c axes all have two abrupt drops at 154 K and 110 K but show significant anisotropy. Moreover, it is noteworthy that the resistivity of the Ti4O7 single crystal in the high-temperature (HT) phase (T > 154 K) exhibits different behavior along different axes, exhibiting semiconducting behavior along the b axis but unusual metallic behavior along the a and c axes, in which the resistivity is proportional to T instead of T2. Therefore, our results apparently do not support the Fermi liquid model proposed in the literature.

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1. INTRODUCTION Ti4O7 has attracted considerable attention over the past several decades owing to its peculiar properties, such as two consecutive phase transitions and puzzling electronic state.1–6 In addition, Ti4O7 also has broad application in the field of solar cells, fuel cells, sewage treatment, and thermoelectric materials owing to its high electronic conductivity, strong visible light absorption, excellent electrochemical corrosion resistance, favorable thermoelectric properties, and environmental compatibility.7–11 Ti4O7 belongs to the TinO2n−1 family, known as the Magnéli phase, consisting of a two-dimensional titanium oxide octahedral plane.1 Ti4O7 is a mixed valence compound with Ti3+ and Ti4+, which is believed to be responsible for the existence of two structural phase transitions at low temperature.5 Along with the structural phase transitions, the transport behavior of Ti4O7 also changes significantly. One is from metal to semiconductor at 𝑇𝑐1  149–155 K, and the other is from semiconductor to semiconductor at 𝑇𝑐2  120–140 K.1–5 The resistivity of Ti4O7 single crystal can change by six orders of magnitude after these two phase transitions.1,5,6 However, there remains some confusion about the nature of the electronic state in Ti4O7.4,12 In the low-temperature phase (T < 140 K), Ti4O7 is a charge-ordered Mott insulator and the local 3d electrons will form Ti3+–Ti3+ bipolaron pairs.3,5 In the intermediate-temperature range, it is usually considered that the Ti3+–Ti3+ pairs still exist in Ti4O7 but without long-range order, which is similar to the bipolaron liquid state. In the high-temperature (HT) phase (T > 154 K), the 3d electrons are believed to be delocalized, and the resistivity exhibits metallic behavior, as evident from early transport data from Bartholomew and Frankl.1 Later, Inglis et al. found that the conductance of Ti4O7 measured in different current directions showed different transport behaviors.6 They declared that the anisotropic conductance of Ti4O7 is an intrinsic behavior, but unfortunately they did not point out the specific crystallographic direction because their crystal was too small to be oriented. In addition, Ti 2p–3d resonant photoemission spectroscopy (PES) and hard X-ray PES show a clear metallic Fermi edge, indicating the Fermi liquid nature of Ti4O7 in the HT phase,4 while Abbate et al. did not observe a clear Fermi edge in the HT phase by PES.3 Therefore, there is no conclusive picture about the nature of the HT phase, and it is necessary to grow large-size Ti4O7 single crystals of high quality for clarifying this issue by accurate electric transport measurement. Chemical vapor transport (CVT) has been the main method used to grow Ti4O7 single crystals for decades.2,3,5,13–16 However, there are some shortcomings in Ti4O7 single crystal grown by CVT. First, the single crystals grown by CVT contain inevitably impurities (e.g., TeCl4 and NH4Cl) owing to the transport medium. Second, typical samples grown by CVT are prisms of dimensions 0.5 mm × 0.5 mm × 3 mm,16 or platelets of dimensions 4 mm × 0.4 mm × 0.1 mm,5 both of which are too small to be orientated. Therefore, it is difficult to study the anisotropic transport


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property. Last, most Ti4O7 crystals large enough for electrical measurement are invariably twinned, which is harmful to the electrical transport measurement. Therefore, it is necessary to explore a new method for growing large-size Ti4O7 single crystals of high quality, such as the crucible-free optical floating-zone method. Unfortunately, according to the thermodynamic phase diagram of Ti4O7 (Figure 1a),17 it will decompose into Ti3O5 and Ti5O9 at temperatures >1923 K, which is lower than the melting point. Consequently, it is very difficult to grow Ti4O7 crystals from melt directly. Taguchi et al. briefly mentioned the growth of Ti4O7 single crystals by an optical floating-zone method, but no details of the crystal growth were given.4 In this paper, we present the successful growth of the high-quality Ti4O7 single crystals by the optical floating-zone technique. We improved the temperature gradient at the solid–liquid interface by employing a high seed rod rotation rate, thereby suppressing the decomposition of Ti4O7 at high temperature. Then, we obtained a high-quality Ti4O7 crystal of 4 mm in diameter and 40 mm in length under Ar atmosphere and a 35 rpm seed rod rotation rate. The anisotropic resistance and anomalous transport behavior suggest that the electronic state of Ti4O7 at high temperature should not be traditional Fermi liquid. 2. EXPERIMENTAL DETAILS As the first step of crystal growth, the feed and seed rods of polycrystalline Ti4O7 were prepared by a solid state reaction. TiO2 (99.9%, Alfa) and Ti (99.99%, Alfa) with a 7:1 molar ratio were ground together and pressed into cylindrical rods of 5 mm in diameter and 70 mm in length under 120 MPa hydrostatic pressure. The feed and seed rods were calcined at 1000 °C in a vacuum-sealed quartz tube (PO2 ~ 10-5 Pa) for 24 h to prevent Ti4O7 from oxidizing at high temperature. Single crystal growth was performed using the floating-zone method in a image furnace with two ellipsoidal mirrors (IR Image Furnace G3, Quantum Design Japan). In order to control the oxygen content in the growing atmosphere, the quartz tube in the furnace was first purged with a vacuum pump and then was evacuated and refilled by pure argon several times. The oxygen content in the quartz tube can then be controlled to Θ𝐷 (where Θ𝐷 is the Debye temperature). Nevertheless, the Debye temperature of Ti4O7 is 493 K,5 which is much higher than our measurement temperature. Therefore, Ti4O7 at high temperature is also not a classic metal as described by a Fermi gas. The anomalous resistance behavior of Ti4O7 single crystals in their HT phase reminds us of copper-based high temperature superconductors. For example, at the best doping concentration, Bi2+xSr2−yCuO6+δ shows metallic behavior in the ab plane and the resistance increases linearly with increasing temperature in the normal state. It also shows semiconducting behavior along the c axis and four orders of magnitude greater resistivity than that in the ab plane.22 In addition, the linear coefficient (𝛽) and



the residual resistivity (𝜌0) fitted by 𝜌 = 𝜌0 +𝛽𝑇 for Ti4O7 along the a axis are 𝜌𝑎0 = 27.4 μΩ cm and 𝛽𝑎 = 0.61 μΩ cm/K, respectively, which are of the same order of magnitude as in Bi2+xSr2−yCuO6+δ.22 The unusual resistance in copper-based high temperature superconductors could be interpreted by using two-dimensional Luttinger liquid theory.23 Therefore, the anomalous resistivity behavior of Ti4O7 in the HT phase looks similar to that of high temperature superconductors in their normal state, suggesting that it does not agree with the Fermi liquid model reported before. However, further research should be pursued, such as: Hall coefficient and thermoelectricity, to determine the intrinsic transport mechanism of Ti4O7 in the HT phase.


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4. CONCLUSION In this paper, we overcome the decomposition of Ti4O7 at high temperature and grow high-quality Ti4O7 single crystals of large size by the optical floating-zone technique. We found that pure Ar atmosphere and an appropriate seed rod rotation rate (35 rpm) contribute to obtaining high-quality Ti4O7 single crystal. The electric resistance of Ti4O7 single crystals exhibits significant anisotropy and a very peculiar behavior in the HT phase, both of which are in contradiction to the previously reported Fermi liquid model for Ti4O7.



AUTHOR INFORMATION Corresponding Author Ming-Xue Huo: [email protected] Xian-Jie Wang: [email protected] Yu Sui: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank Professor Jian-Shi Zhou for helpful discussions. This work is supported by the Natural Science Foundation of China (No. 51472064).


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REFERENCES (1) Bartholomew, R. F.; Frankl, D. R. Electrical Properties of Some Titanium Oxides. Phys. Rev. 1969, 187, 828–833. (2) Schlenker, C.; Lakkis, S.; Coey, J. M. D.; Marezio, M. Heat Capacity and Metal-Insulator Transitions in Ti4O7 Single Crystals. Phys. Rev. Lett. 1974, 32, 1318– 1321. (3) Abbate, M.; Potze, R.; Sawatzky, G. A.; Schlenker, C.; Lin, H. J.; Tjeng, L. H.; Chen, C. T.; Teehan, D.; Turner, T. S. Changes in the electronic structure of Ti4O7 across the semiconductor-semiconductor-metal transitions. Phys. Rev. B. 1995, 51, 10150. (4) Taguchi, M.; Chainani, A.; Matsunami, M.; Eguchi, R.; Takata, Y.; Yabashi, M.; Tamasaku, K.; Nishino, Y.; Ishikawa, T.; Tsuda, S.; Watanabe, S.; Chen, C. T.; Senba, Y.; Ohashi, H.; Fujiwara, K.; Nakamura, Y.; Takagi, H.; Shin, S. Anomalous State Sandwiched between Fermi Liquid and Charge Ordered Mott-Insulating Phases of Ti4O7. Phys. Rev. Lett. 2010, 104, 106401. (5) Lakkis, S.; Schlenker, C.; Chakraverty, B. K.; Buder, R.; Marezio, M. Metal-insulator transitions in Ti4O7 single crystals: crystal characterization, specific heat, and electron paramagnetic resonance. Phys. Rev. B. 1976, 14, 1429. (6) Inglis, A. D.; Page, Y. Le.; Strobel, P.; Hurd, C. M. Electrical conductance of crystalline TinO2n−1 for n=4–9. J. Phys. C: Solid State Phys. 1983, 16, 317. (7) Kumar, S. G.; Devi, L. G. Review on modified TiO2 photocatalysis under UV/visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics. J. Phys. Chem. A 2011, 115, 13211–13241. (8) You, S.; Liu, B.; Gao, Y. F.; Wang, Y.; Chuyang, Y. T.; Huang, Y. B.; Ren, N. Q. Monolithic Porous Magnéli-phase Ti4O7, for Electro-oxidation Treatment of Industrial Wastewater. Electrochim. Acta. 2016, 214, 326–335. (9) Chisaka, M.; Ando, Y.; Yamamoto, Y.; Itagaki, N. A carbon-support-free titanium oxynitride catalyst for proton exchange membrane fuel cell cathodes. Electrochim. Acta. 2016, 214, 165–172. (10) Wang, G.; Liu, Y.; Ye, J.; Qiu, W.; Ma, S.; An, X. Fabrication of rod-like Ti4O7, with high conductivity by molten salt synthesis. Mater. Lett. 2017, 186, 361–363. (11) Kieslich, G.; Tremel, W. Magnéli oxides as promising n-type thermoelectrics. AIMS Mater. Sci. 2014, 1, 184–190. (12) Liborio, L.; Mallia, G.; Harrison, N. Electronic structure of the Ti4O7 Magnéli phase. Phys. Rev. B. 2009, 79, 245133. (13) Mercier, J.; Lakkis, S. Preparation of titanium lower oxides single crystals by chemical transport reaction. J. Cryst. Growth. 1973, 20, 195–201. (14) Hong, S. H. Crystal Growth of Some Intermediate Titanium Oxide Phases gamma-Ti3O5, beta-Ti3O5, Ti4O7 and Ti2O3 by Chemical Transport Reactions. Acta Chem. Scand., Ser. A. 1982, 36, 207–217.



(15) Calestani, D.; Licci, F.; Kopnin, E.; Calestani, G.; Gauzzi, A.; Bolzoni, F.; Besagni, T.; Boffa, V.; Marezio, M. Preparation and characterization of powders and crystals of Vn−xTixO2n−1 Magneli oxides. Cryst. Res. Technol. 2005, 40, 1067–1071. (16) Strobel, P.; Page, Y. Le. Crystal growth of TinO2n−1 oxides (n=2 to 9). J. Mater. Sci. 1982, 17, 2424–2430. (17) Eriksson, G.; Pelton, A. D. Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the MnO-TiO2, MgO-TiO2, FeO-TiO2, Ti2O3-TiO2, Na2O-TiO2, and K2O-TiO2 systems. Metall. Mater. Trans. B. 1993, 24, 795–805. (18) Itoh, K.; Marumo, F.; Kuwano, Y. β-barium borate single crystal grown by a direct Czochralski method. J. Cryst. Growth. 1990, 106, 728–731. (19) Kozuki, Y. Itoh, M. Metastable crystal growth of the low temperature phase of barium metaborate from the melt. J. Cryst. Growth. 1991, 114, 683–686. (20) Parker, R. L. Crystal growth mechanisms: energetics, kinetics, and transport. Solid State Phys. 1970, 25, 151–299. (21) Marezio, M.; McWhan, D. B.; Dernier, P. D.; Remeika, J. P. Structural aspects of the metal-insulator transitions in Ti4O7. J. Solid State Chem. 1973, 6, 213–221. (22) Martin, S.; Fiory, A. T.; Fleming, R. M.; Schneemeyer, L.F.; Waszczak, J. V. Normal-state transport properties of Bi2+xSr2−yCuO6+δ crystals. Phys. Rev. B. 1990, 41, 846. (23) Anderson, P. W. Hall effect in the two-dimensional Luttinger liquid. Phys. Rev. Lett. 1991, 67, 2092.


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Figure 1. (a) Phase diagram for the TiO1.5–TiO2 binary system in an argon atmosphere (adapted from Ref. 17). (b) Schematic diagram based on our growth results.



Figure 2. Powder XRD patterns of (a) a Ti4O7 polycrystalline rod, (b) the sample in an argon atmosphere containing 3% hydrogen, (c) the sample in an argon atmosphere without evacuating, (d) the sample at a seed rod rotation rate of 20 rpm in a highly pure argon atmosphere, and (e) the sample at a seed rod rotation rate of 40 rpm in a highly pure argon atmosphere.


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Figure 3. Schematic diagram of the thermal boundary layer. 𝑇𝑚 is the temperature at the solid–liquid interface; 𝑇𝑏 is the temperature inside the melt; 𝜔 is the rotation rate of the seed rod; 𝛿𝑇 is the thickness of the thermal boundary layer. In general, 𝑇𝑏 > 𝑇𝑚 and a thermal boundary layer is used approximately to describe the transition region. The thickness is a function of rotation rate. The solid blue line is the temperature distribution near the solid–liquid interface when the rotation rate of the seed rod is 𝜔0 (lower rotation rate). The red dotted line is the temperature distribution near the solid–liquid interface when the rotation rate of the seed rod is 𝜔1 (higher rotation rate). The slope of the line represents the temperature gradient.



Figure 4. Cross-sectional morphology of Ti4O7 single crystals at (a) 40 rpm and (b) 35 rpm.


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Figure 5. The oddity in the Ti4O7 crystal growth process (a) before the melt begins to spontaneously flow downward, (b) when the melt begins to spontaneously flow downward and the raised pleat begins to occur, (c) at the end of the raised pleat, and (d) when a stable floating zone appears after the strange phenomenon. The raised pleat is noted by the green ellipses. The component of the sample is a mixture of the primary phase for Ti3O5 before the melt begins to spontaneously flow downward, while it is pure Ti4O7 in the stable growth part.



Figure 6. (a) Image of the as-prepared Ti4O7 single crystal grown using the floating-zone method. (b) Photograph of the orientational Ti4O7 single crystal. (c) Laue picture taken of the Ti4O7 single crystal along the c-axis direction.


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Figure 7. (a) Rietveld refinement results of powder XRD measurements on powders obtained from our crushed floating-zone-grown single crystals. (b) X-ray rocking scan curve of the (0 0 2) Bragg peak. The full width at half maximum of 0.06° indicates the high quality of the sample.



Figure 8. (a) Temperature dependence of the electric resistance of the as-grown Ti4O7 single crystal. (b)–(d) Temperature dependence of the electric resistance of the as-grown Ti4O7 single crystal along the a, b, and c axes in the HT phase, respectively. The red line is a linear fitting curve.


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For Table of Contents Use Only

Large Size Single Crystal Growth of Ti4O7 by the Floating-Zone Method Qing-Yuan Liu,† Zi-Yi Liu,† Xue-Bo Zhou,† Zhi-Guo Liu,† Ming-Xue Huo,*,‡ Xian-Jie Wang,*,† and Yu Sui*,† †Department

of Physics, Harbin Institute of Technology, Harbin 150001, China ‡Research Center of Basic Space Science, Harbin Institute of Technology, Harbin 150001, China

Synopsis Ti4O7 has received attention because of its unique physical properties. However, detailed study of its peculiar anisotropic transport properties has been hampered by the lack of sizable single crystals. Here, we report the growth of a centimeter-sized single crystal using the floating-zone technique. The resistance of our crystal exhibits strong anisotropy and inherently complex behavior.


Large Size Single Crystal Growth of Ti4O7 by the Floating-Zone

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