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Structure of single-crystal rutile (TiO2) prepared by high-temperature ultracentrifugation Tsutomu Mashimo, Rabaya Bagum, Yudai Ogata, Makoto Tokuda, Maki Okube, Kazumasa Sugiyama, Yoshiaki Kinemuchi, Hiroshi Isobe, and Akira Yoshiasa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01818 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 4, 2017
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Crystal Growth & Design
Structure of single-crystal rutile (TiO2) prepared by high-temperature ultracentrifugation Tsutomu Mashimo*†, Rabaya Bagum†, Yudai Ogata†, Makoto Tokuda†, Maki Okube‡, Kazumasa Sugiyama‡, Yoshiaki Kinemuchi§, Hiroshi Isobe＆, Akira Yoshiasa＆ †
Institute of Pulsed Power Science, Kumamoto University, Kumamoto 860-8555, Japan
‡
Institute of Materials Science, Tohoku University, Sendai 980-8577, Japan
§
National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan
＆
Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan
ABSTRACT: We report the preparation of single-crystal rutile (TiO2) with a unique structure prepared by ultracentrifugation at high temperature (strong gravitational field). Single-crystal rutile was subjected along the c-axis direction to a gravitational field of 0.4 × 106 G at 400 °C, and the uniquely structured single-crystal rutile, which did not conform to Pauling’s third rule, was quenched at ambient conditions. The anisotropy (a/c ratio) of the tetragonal phase increased by 2%. The (Ti–Oa)/(Ti–Ob) and Os/Ou ratios increased by 1.6% and 3%, respectively, and approached 1; Ti–Oa and Ti–Ob are the two Ti–O interatomic distances, and Os and Ou are the shared edge Oa–Oa, and the unshared edge Oa–Ob, respectively. This means that the TiO6 octahedral group became isotropic. Os was expanded from its normal size, in contradiction to the usual laws, by the strong gravitational force. Such a structural change has not previously been
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achieved under high- pressure or high-temperature conditions, and may be related to structural stabilization induced by a unique uniaxially distordted crystalline state under ultracentrifugation. Ab initio simulations supported the experimental results.
Titanium oxide-based materials have attracted great interest and been intensively investigated because of their interesting optical, dielectric, catalytic, thermal, and mechanical properties.1-6 TiO2 exists in three crystalline forms: anatase, rutile, and brookite. Rutile is the most common form in nature and is generally considered to be the stable phase under ambient conditions.7 The rutile crystal structure consists of Ti atoms octahedrally surrounded by oxygen atoms, and each oxygen has three Ti atoms as neighbors; however, the structural groups are linked in different ways. In these crystalline forms of TiO2, the mean Ti–O bond length is around 1.90–2.00 Å and the O–O bond length is between 2.50 and 3.00 Å, with the shorter lengths corresponding to edgeshared oxygen.8 The density of crystalline TiO2 is 4.23 g/cm3. Rutile is the thermodynamically stable phase and its band gap energy (3.0 eV) is lower than that of the anatase phase (3.2 eV). It is used as the main white pigment in paints and cosmetic products because it is inert, nontoxic, and has high light-scattering and refractive indices. Gravity is a field state variable, like magnetic and electric fields, which acts directly on atoms in materials through a body force, whereas pressure and temperature are thermodynamic variables that affect atoms statistically. Schematic diagrams of crystal structures under pressure and in a gravitational field are shown in Fig. 1. Under pressure, the lattice shrinks isotropically according to the equation of state of matter. In contrast, in a strong gravitational field (~106 G), the heavier atoms in a compound crystal are subjected to a stronger force in the gravitational direction than are lighter atoms because of the different body forces acting on the respective atoms owing to their different atomic weights. If the position of the crystal is restrained by a
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wall, heavy atoms are forced in the gravitational direction and light atoms are forced in the opposite direction; as a result, a unique crystalline state, which is uniaxially distorted, appears, as shown in Fig. 1. A strong gravitational field has two effects on materials. First, if the gravitational potential related to the relative body force is comparable to the chemical potential, then gravity-induced diffusion (sedimentation of atoms) occurs.9 We developed a hightemperature ultracentrifuge that can generate strong gravitational fields, in excess of 106 G, at temperatures >500 °C for extended periods.10 We have used this ultracentrifuge to study the gravity-induced diffusion of atoms in alloys11,12 and the separation of isotopes.13
Figure 1. Schematic diagram of crystal structures of compound under high pressure and in a strong gravitational field where the crystal position is restrained by a wall.
Secondly, the unique uniaxially distorted crystalline state can induce structural changes in compounds other than those caused by high pressure and high temperature, enabling formation of a unique metastable phase. In this study, to examine the structural changes under a strong gravitational field, we performed high-temperature ultracentrifugation experiments on a rutile TiO2 single crystal along the c-axis. Single-crystal rutile with a unique structure was quenched at ambient conditions.
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Single crystals were grown using a flame fusion method and were provided by the Nakazumi Crystal Co., Ltd. These crystals were cut into circular plates of diameter 5 mm and thickness 1 mm, with a (001) crystal plane. The plate-shaped sample was set on a sapphire plate in a stainless-steel SUS304 capsule, and put in a rotor of the ultracentrifuge. The gravitational field was applied along the c-axis of the crystal; the maximum distance from the rotor axis was 35.5 mm. The gravitational force on the sample at a rotational velocity of 100 000 rpm was (0.39– 0.40) × 106 G in a vacuum. The rotor was radiatively heated to 400 °C using a hot carbon hollow cylinder, which in turn was heated by a high-frequency heating system.10 The duration was 24 h. After the ultracentrifugation experiment, the electrical heating source was removed, and the rotor cooled to room temperature in air in about 2 h. Rotation was then stopped within about 5 min, and the vacuum in the chamber was broken. Single-crystal X-ray diffraction, which is one of the most reliable structure analysis techniques, was performed using a four-circle diffractometer at the BL-10A beam line of the Photon Factory, Tsukuba, Japan, with monochromated synchrotron X-ray radiation. The beam size was 1.0 mm and a scintillation detector was used. Small particles of diameter less than 0.2 mm were picked out from the upper parts of the plate-shaped gravity-induced and initial single crystals, and set in glass fiber using epoxy glue. The extinction rule confirmed that each specimen had the P42/mnm space group. In total, 408 and 224 reflections were recorded for gravity-induced and initial crystals, respectively, and averaged in Laue symmetry 4/mmm to give 43 and 63 independent reflections, with |Fo| ≥ 3σ(|Fo|) used for the structure refinements. The number of refined parameters was nine. The structure refinements were performed using the fullmatrix least-squares program RADY.14 The final reliability indices (R = Σ||Fo| − |Fc||/Σ|Fo|) converged to R = 0.0462 and 0.0147. After the final refinement, no significant residual electron
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density was detected in the difference-Fourier maps. The values of the reliability indices R, i.e., 4.62% and 1.47%, were sufficient. The analysis had sufficient accuracy, comparable to that of our previous temperature dependence structural analyses work.15 The larger R value for the gravity-induced specimen presumably arises from the heterogeneity of local statistical atomic arrangements remaining in the quenched material after removal of the strong gravitational field. The compositions of the two specimens were determined by electron probe microanalysis using a JXA-800R electron microprobe (Japan Electron Optics Laboratories Co, Ltd.) The X-ray crystallographic data for the gravity-induced and ambient-state samples are summarized in Tables 1 (lattice parameters, etc.) and 2 (interatomic distances and bond length ratios). Figure 2(a) shows comparisons of the a/c ratios of the ultracentrifuged sample with those obtained from high-pressure16,17 and high-temperature15 variations. The ratio is clearly different from the standard one and those obtained under high pressure or high temperature. The axial ratio a/c decreased with increasing pressure, i.e., the a-axis was more compressible than the caxis. This may be caused by metal-to-metal repulsion parallel to the c-axis. However, for the ultracentrifuged sample, the lattice constant c decreased (0.3%) and a increased (0.05%); the anisotropy, i.e., the a/c ratio, increased by 2%, although this ratio decreased under high pressure and high temperature. We confirmed the similar lattice parameter changes at several parts of the ultracentrifuged sample and of the other samples. The effect of pressure during ultracentrifugation was negligible in the upper part of the gravity-induced sample (
