Article pubs.acs.org/crystal
Structural Phase Transition between γ‑Ti3O5 and δ‑Ti3O5 by Breaking of a One-Dimensionally Conducting Pathway Kenji Tanaka,† Tomomichi Nasu,† Yasuto Miyamoto,† Noriaki Ozaki,† Shu Tanaka,† Toshiaki Nagata,† Fumiyoshi Hakoe,† Marie Yoshikiyo,† Kosuke Nakagawa,† Yoshikazu Umeta,† Kenta Imoto,† Hiroko Tokoro,†,‡ Asuka Namai,† and Shin-ichi Ohkoshi*,†,§ †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Division of Materials Science, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan § CREST, Japan Science and Technology Agency (JST), K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ‡
S Supporting Information *
ABSTRACT: The phase transition between gamma-trititanium-pentoxide (γ-Ti3O5) and delta-trititanium-pentoxide (δTi3O5) was clarified from both experimental and theoretical viewpoints. With decreasing temperature, the monoclinic I2/c crystal structure of γ-Ti3O5 was found to switch to a monoclinic P2/a crystal structure of δ-Ti3O5 due to lowering of symmetry. Electrical conductivity (σ) measurement shows that γ-Ti3O5 behaves like a metallic conductor with a σ value of 4.7 S cm−1 at 320 K, while δ-Ti3O5 shows a semiconductive property with a σ value of 2.5 × 10−5 S cm−1 at 70 K. Optical measurement also supports that γ-Ti3O5 is a metallic conductor, while δ-Ti3O5 is a semiconductor with a band gap of 0.07 eV. First-principles calculations show that γ-Ti3O5 is a metallic conductor, and the energy state on the Fermi energy is composed of the 3d orbital of Ti and 2p orbital of O with onedimensional linkage along the crystallographic c-axis. On the contrary, δ-Ti3O5 has a band gap, and the energy state around the Fermi energy is split into the valence band and the conduction band, which are assigned to the lower and upper Hubbard bands, respectively. Thus, the phase transition between γ-Ti3O5 and δ-Ti3O5 is caused by breaking of a one-dimensionally conducting pathway due to a Mott−Hubbard metal−insulator phase transition.
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electrical property, and optical property of δ-Ti3O5 has not been clarified yet. In the present work, we determine the precise atomic positions of δ-Ti3O5 in addition to those of γTi3O5 and investigate the electrical conductivities and optical properties. Furthermore, the electronic structures of γ-Ti3O5 and δ-Ti3O5 are calculated by first-principles calculations, and the mechanism of this phase transition is discussed.
INTRODUCTION Titanium dioxide TiO2 has been widely used in our life, for example, white pigments, cosmetic materials, medicines, capacitors, memory devices, and photocatalysts,1−14 which is composed of Ti4+ (3d0) and O2−. On the other hand, titanium oxides including Ti3+ (3d1, S = 1/2), such as Ti2O3,15−19 Ti3O5,20−29 and Ti4O7,30−34 are also attractive functional materials. For example, Ti2O3 has been used as a black pigment and heat absorbing material. Titanium oxides including Ti3+ show black or dark blue color due to d−d transition on Ti3+, and therefore, they are often called black titanium oxide. In the case of Ti3O5, a metal−semiconductor phase transition between α- and β-phases is known to occur at 460 K.20,21 Very recently, our group found a new polymorph of Ti3O5, λTi3O5.23 This λ-Ti3O5 exhibits a light-induced phase transition to β-Ti3O5 at room temperature. A light-induced phase transition at room temperature is the first to be observed among metal oxides.23 As another thermal phase transition of Ti3O5, a phase transition from γ-Ti3O5 to δ-Ti3O5 with decreasing temperature is known.27,28 However, there are few reports concerning this phase transition between γ-Ti3O5 and δ-Ti3O5, and information about the precise atomic positions, © XXXX American Chemical Society
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EXPERIMENTAL SECTION
γ-Ti3O5 was prepared by calcination of rutile-type TiO2 under reduction conditions. In the sintering process, TiO2 was heated from room temperature to 1237 K with 150 K h−1 speed, held at 1237 K for 5 h, and then cooled to room temperature with 150 K h−1 speed under H2 condition with a flow rate of 3 dm3 min−1. Elemental analysis was conducted using an Agilent 7700x inductively coupled plasma mass spectrometer (ICP-MS). Powder X-ray diffraction (PXRD) measurements were conducted using a Rigaku Ultima-IV with Cu Kα radiation (λ = 1.5418 Å). Rietveld analyses were performed using RIETAN-FP35 and PDXL programs. Electrical conductivity was measured by the fourReceived: September 6, 2014 Revised: November 15, 2014
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DOI: 10.1021/cg5013439 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Crystal Growth & Design
Figure 1. (a) PXRD pattern of γ-Ti3O5 at 300 K and Rietveld analysis (left): red dots, black line, and gray line are the observed pattern, the calculated pattern, and the difference, respectively, and dark red bars represent the calculated positions of the Bragg reflections of γ-Ti3O5. Crystal structure of γ-Ti3O5 (right): yellow, pink, and red balls represent Ti(1), Ti(2), and O atoms, respectively. (b) Temperature dependence of PXRD patterns from 300 to 200 K (left), and temperature evolution of the phase fractions of γ-Ti3O5 and δ-Ti3O5 (right). (c) PXRD pattern of δ-Ti3O5 at 150 K and Rietveld analysis (left): blue dots, black line, and gray line are the observed pattern, the calculated pattern, and the difference, respectively, and dark blue bars represent the calculated positions of the Bragg reflections of δ-Ti3O5. Crystal structure of δ-Ti3O5 (right): cyan, green, light blue, and blue balls represent Ti(1a), Ti(1b), Ti(2), and O atoms, respectively.
Table S2. γ-Ti3O5 has two nonequivalent Ti sites, Ti(1) and Ti(2), and three nonequivalent O sites, O(1)−O(3). Ti(1)O6 cluster has a distorted octahedron coordination geometry (Ti− O distance: 1.889−2.176 Å, standard deviation (sd): 0.097 Å), while Ti(2)O6 has a regular octahedron coordination geometry (1.998−2.032 Å, sd: 0.017 Å). The lattice parameters are consistent with those reported in the previous paper.28 As temperature decreases below 240 K, the PXRD peaks of γTi3O5 decrease, and additional new peaks appear (Figure 1b), which indicates that a structural change occurs. The lattice constants at each temperature are shown in Table S3. In the light of the previous paper,29 the low temperature phase is considered as δ-Ti3O5. The phase fraction versus temperature plot (Figure 1b) indicates that the temperature where the phase fractions of γ-Ti3O5 and δ-Ti3O5 become equal (Tp) is evaluated to be 237 K. Rietveld analysis for the PXRD pattern
probe method in a Quantum Design PPMS 6000. The infrared (IR) spectra were measured using a JASCO FIR-6100. The magnetic measurement was carried out using a superconducting quantum interference device magnetometer, a Quantum Design MPMS-7. Firstprinciples calculations were conducted using Vienna ab initio simulation package (VASP).36,37
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RESULTS AND DISCUSSION Inductively coupled plasma mass spectrometry results show that the composition of the obtained material is Ti3.00(4)O5.00(7); Calc.: Ti, 64.22%. Found: Ti, 64.27(89)%. PXRD pattern with Rietveld analysis shows that the crystal structure of γ-Ti3O5 at 300 K is a monoclinic structure (space group: I2/c) with lattice constants of a = 9.9695(7) Å, b = 5.0739(3) Å, c = 7.1817(5) Å, and β = 109.8633(6)° (Figure 1a, Figure S1, and Table S1, Supporting Information). The atomic positions are listed in B
DOI: 10.1021/cg5013439 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Crystal Growth & Design measured at 150 K is shown in Figure 1c and Table S1. The crystal structure at 150 K is a monoclinic structure (space group: P2/a) with lattice parameters of a = 9.9651(7) Å, b = 5.0604(4) Å, c = 7.2114(5) Å, and β = 109.3324(9)° (Figures 1c and S2). In δ-Ti3O5, the Ti(1) site of γ-Ti3O5 splits into two different Ti sites, Ti(1a) and Ti(1b), resulting in three nonequivalent Ti sites (Ti(1a), Ti(1b), and Ti(2)) and six nonequivalent O sites (O(1a)−O(3b)). The atomic positions are listed in Table S2. Ti(1a)O6 and Ti(1b)O6 clusters have distorted octahedron coordination geometries (Ti(1a): 1.87− 2.17 Å, sd: 0.11 Å, Ti(1b): 1.89−2.19 Å, sd: 0.10 Å), and Ti(2)O6 has a regular octahedron coordination geometry (1.98−2.08 Å, sd: 0.04 Å). Such precise atomic positions of δTi3O5 are the first to be confirmed. The valence state for each Ti site is estimated from the empirical relationship between bond length and valence states, which indicates that the valence states for Ti(1) and Ti(2) in γ-Ti3O5 are +3.36 and +3.30, and the valence states for Ti(11), Ti(12), and Ti(2) in δ-Ti3O5, are +3.66, +3.16, and +3.20, respectively. Temperature dependence of the electrical conductivity measured by the four-probe method is shown in Figure 2. At
Figure 3. IR spectra of γ-Ti3O5 at 300 K (red) and δ-Ti3O5 at 150 K (blue). Inset is the fitting of the IR spectra.
Figure 4. Temperature dependence of magnetic susceptibility measured under external magnetic field of 5000 Oe.
Figure 2. Temperature dependence of electrical conductivity. Inset is the temperature dependence of electrical conductivity in logarithmic scale.
formation from δ-Ti3O5 to γ-Ti3O5 was observed in the warming process. Figure 5a shows the density of states (DOS) of γ-Ti3O5 and δ-Ti3O5 near the Fermi energy, from −1.5 eV to +2.5 eV. In the DOS of γ-Ti3O5, the band gap does not exist at the Fermi energy, implying that γ-Ti3O5 is a metallic conductor. The band structure around the Fermi energy is shown in Figure S3, Supporting Information. From the Γ point of the energy state in the band structure, a charge density map of γ-Ti3O5 is illustrated. In the energy state of γ-Ti3O5 above the Fermi energy, the 3d orbital of Ti(2) and 2p orbital of O(3) are infinitely linked one-dimensionally along the crystallographic caxis (Figure 5b). The charge is also delocalized between Ti(2) and Ti(1). On the contrary, in the case of δ-Ti3O5, a band gap is present. The charge density maps of the conduction band bottom and the valence band top are shown in Figure 5b, showing that the energy state of γ-Ti3O5 above the Fermi energy has split into two energy states in δ-Ti3O5. The conduction band and the valence band are considered as the upper Hubbard band and the lower Hubbard band, respectively. In both of these two states of δ-Ti3O5, the conducting pathway is disconnected, consistent with the drop of electrical conductivity. Thus, the phase transition between γ-
320 K, the electrical conductivity (σ) of γ-Ti3O5 was 4.7 S cm−1. With decreasing temperature, the conductivity abruptly drops around Tp. For example, the σ value of δ-Ti3O5 at 70 K was 2.5 × 10−5 S cm−1. These two σ values belong to the category of semiconductor, but the values of γ-Ti3O5 and δTi 3 O 5 are close to metallic conductor and insulator, respectively. Figure 3 shows the IR spectra of γ-Ti3O5 at 300 K and δTi3O5 at 150 K. The IR spectrum of γ-Ti3O5 is flat over the entire energy range, whereas that of δ-Ti3O5 decreases in the low energy region, suggesting the absence and the presence of a band gap, respectively. The square root of hνA versus energy plots of δ-Ti3O5 is linear; i.e., (hνA)1/2 ∝ hν − Eg, where hν, A, and Eg are photon energy, IR absorbance, and band gap, respectively, implying that δ-Ti3O5 is an indirect transition semiconductor with a band gap of 0.07 eV (Figure 3, inset). With decreasing temperature, the magnetic susceptibility (χ) dropped around Tp (Figure 4), which corresponds to the previous report.27 This χ decrease is understood by the transition from a Pauli paramagnetic state to a nonmagnetic semiconducting state. Furthermore, the reverse phase transC
DOI: 10.1021/cg5013439 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
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Article
ASSOCIATED CONTENT
S Supporting Information *
Computational details, crystal structures, crystallographic data, atomic positions of γ-Ti3O5 and δ-Ti3O5, lattice constants of γTi3O5 and δ-Ti3O5 at different temperatures, and band structures of γ-Ti3O5 and δ-Ti3O5. This material is available free of charge via the Internet at http://pubs.acs.org. X-ray crystallographic information files (CIF) are available for γTi3O5 and δ-Ti3O5. Crystallographic data for the structure of δTi3O5 has been deposited with Cambridge Crystallographic Data Centre as supplementary publication No. CCDC1004604.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Mr. K. Chiba of Ryoka Systems Inc. for the valuable advice and discussions. This work was supported by the CREST project of JST, and APSA from MEXT. We also recognize the Cryogenic Research Center, The University of Tokyo, and the Center for Nano Lithography & Analysis, The University of Tokyo, which are supported by MEXT. M.Y. is grateful to the ALPS program. T.N., Y.M., and Y.U. are grateful to the MERIT program. H.T., A.N., and K.N. are thankful to the Grant-in-Aid for Young Scientists (A), (B), and the Research Activity Start-up, respectively, and N.O., S.T., and M.Y. are grateful for a Research Fellowship for Young Scientists of JSPS.
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Figure 5. (a) DOS of γ-Ti3O5 and δ-Ti3O5. Green band indicates the energy state at the Fermi energy (EF) of γ-Ti3O5, and red and blue bands indicate the lowest energy state in the conduction band and the highest energy state in the valence band, respectively, of δ-Ti3O5. (b) Charge density map of the conduction band bottom of γ-Ti3O5 with a schematic illustration of the orbitals constructing the one-dimensional conducting pathway along the crystallographic c-axis (right). Charge density maps of the conduction band bottom and the valence band top of δ-Ti3O5, corresponding to the upper and the lower Hubbard bands, respectively (left). Tp is the temperature of the Mott−Hubbard metal−insulator phase transition.
Ti3O5 and δ-Ti3O5 is confirmed to be a Mott−Hubbard metal− insulator phase transition.
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CONCLUSION
In this work, the crystal structures, electrical conductivities, and optical properties of γ-Ti3O5 and δ-Ti3O5 are clarified. γ-Ti3O5 has metallic property, while δ-Ti3O5 is a semiconductor. The phase transition between γ- and δ-Ti3O5 is caused by a Mott− Hubbard metal−insulator phase transition, and the change in the electrical conductivity is caused by disconnection of the conducting pathway in the crystal. The mechanism of the phase transition between γ-Ti3O5 and δ-Ti3O5 is the first to be clarified from both experimental and theoretical approaches. D
DOI: 10.1021/cg5013439 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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DOI: 10.1021/cg5013439 Cryst. Growth Des. XXXX, XXX, XXX−XXX
