Graduate School of Science and Engineering Applied Chemistry

Laboratory of Inorganic Synthetic Chemistry

Aiming for new materials, new properties and new synthetic processes

Staff

KATO Masaki
[Professor]

Acceptable course
Master's degree course	✓
Doctoral degree course	✓

Telephone : +81-774-65-6686
makato@mail.doshisha.ac.jp
Office : SC-207
Database of Researchers

OTA Hiroto
[Associate Professor ]

Acceptable course
Master's degree course
Doctoral degree course

Telephone : +81-774-65-6690
hirota@mail.doshisha.ac.jp
Office : SC-208
Database of Researchers

Research Topics

• Fabrication of magnetic metal/ceramic composites used for electric applications
• Development of oxide thermoelectric materials
• Fabrication of carbon nano fiber (CNF) or carbon nano tube (CNT) dispersed engineering ceramics
• Fabrication of high-tempertaure engineering ceramics usable around 1,500~1,600°C
• Fabrication of dense bio-ceramics using Fused Deposition Modeling (FDM) type 3D printer
• Fabrication of dense ZrO₂-Al₂O₃ based ceramics with high mechanical properties by pressure-less sintering
• Preparation of antibacterial ceramic powders, such as ZnO and TiO₂
• Effect of elemental substitution on magnetic interactions in layered copper oxide
• Effect of elemental substitution on magnetism in indium (In), copper (Cu) oxide with its particularly low-dimensional structure
• Elemental substitution effect and microscopic electronic properties of pyrochlore oxide
• Synthesis and physical properties of iron-based superconductive compounds and properties
• Synthesis and physical properties of titanium oxide as a new transparent electrode

Research Contents (Prof. Ken HIROTA)

High-temperature bending strength σ_b of (B₄C/CNF) composites:
high σ_b is attained at1,400~1,700°C (1,673~1,973 K).

Research Contents (Associate Prof. Masaki KATO)

Superconductivity is a phenomenon whereby perfect diamagnetism (property which internally cancel the external magnetic field) called Meissner effect occurs with zero electric resistivity. The applications are too numerous to mention (e.g. development of ultra-strong magnetic fields, lossless power transmission, linear-motor trains and other modes of transport that use magnetic levitation, power storage, nuclear fusion), but the development of materials that become superconductive at high temperatures remains a major issue. To this end, a fundamental elucidation of the mechanisms regarding emergence of superconductivity is necessary.

Notably, it is becoming clear that the dimensionality of the crystalline structure are closely linked to superconductivity and magnetism since the recent discovery of high temperature oxide superconductors and their related compounds. This strong association between structure and electron properties (conductivity and magnetism) is particularly noteworthy in inorganic compounds including transition-metals, and this stems from a much stronger correlation (strong electron correlation) in solids than in ordinary materials. This is a major topic in properties research of interest from both an experimental and theoretical viewpoint. For example, it can be said that all unique electric and magnetic phenomena in transition-metal compounds (such as materials that transition from metal to insulator at a certain temperature, itinerant electron magnets in which the electrons responsible for electrical conductivity also display magnetism, and heavy electron systems of compounds including rare metals in which the effective mass of electrons increases up to 100-1,000 times than normal) are based on strong electron correlation.

However, research on strong electron correlation in solids is still in the early stages, and theoretical discussion is extremely difficult; thus a fundamental understanding will require consolidation of more experimental knowledge.

Thus, this laboratory synthesizes such electrically and magnetically unique materials. Specifically, we produce layered inorganic ceramic compounds controlled materially and structurally on a nanoscale by introducing various atoms and molecules between layers in layered transition-metal compounds. We then analyze their structure using X-ray diffraction, electron microscope observation, and the like, and evaluate their physical properties using such measurements as magnetic susceptibility/electrical resistivity measurement, nuclear magnetic resonance (NMR) measurement, and neutron diffraction, in order to come to an understanding on a nanoscale of the various phenomena (or quantum criticality) based on superconductivity or electron correlation or the like. The knowledge gleaned is fed back into the synthesis, with the ultimate aim of producing new functional inorganic compounds.

<1> Metal-insulator crossover in transition-metal oxides with pyrochlore-type structure

Pyrochlore oxides constitute a large family of transition metal oxides They have a general formula of A₂B₂O₇, where A is usually a larger cation such as lanthanoid series, and B is a smaller transition metal. Both A and B ions individually form a three dimensional sub-lattice consisting of corner-shared tetrahedra. When the exchange interaction in the sub-lattice is antiferromagnetic, the magnetic moments are expected to be geometrically frustrated. As a result, the system shows a spin-glass behavior which has been found in several pyrochlores or a disordered ground state instead of a magnetically ordered state. Electric and magnetic properties of pyrochlores with B sites of 4d elements are especially attractive, since the 4d electrons generally indicate the variety of localized and itinerant behaviors10-19). For example, the bismuth ruthenate Bi₂Ru₂O₇ is metallic and Pauli paramagnetic with a nearly temperature-independent resistivity⁹⁾, while the rare earth ruthenates Ln₂Ru₂O₇ (Ln represents lanthanoids from Pr to Lu) are all semiconductors with localized magnetic moment on the ruthenium atoms. In order to clarify the transport behaviors of pyrochlore ruthenates near the boundary of the metal-insulator crossover, detailed investigations of the physical properties have been performed for the Ln-substituted system Bi₂-_xLn_xRu₂O₇15). Although Pb₂Ru₂O_7-δ is known to show a Pauli paramagnetism with a metallic behavior similar to Bi₂Ru₂O₇, there have been a few reports on Ln-substituted system Pb_2-xLn_xRu₂O_7-δ. In this article, our studies for the synthesis and physical properties of Pb_2-xEu_xRu₂O_7-δ solid solution are reviewed^21),22). Especially, we report Pb-Eu system from viewpoints of (1) magnetic and electric properties, (2) microscopic structural parameters and (3) strongly-correlated electron system behaviors. In addition, our new results of synthesis under high-pressure oxygen for Pb_2-xCa_xRu₂O_7-δ system are also reported.


Schematic view of pylochlore-type structure with chemical formula of A_2B2O7

<2> Elemental substitution effects on physical properties of Cu-based oxides with low-dimensional structure

Low-dimensional magnetic compounds, in which magnetic atoms form the one- or two-dimensional (1-D or 2-D) structure, usually show a variety of phenomena such as quantum spin effect, spin frustration and superconductivity. These phenomena are caused by the enhancement of quantum fluctuation in low-dimensional structure. Thus, it is a key role to elucidate the physical properties of low-dimensional compounds with the quantum fluctuation effect.
CuGeO₃ consists of 1-D CuO₂ chains formed by the edge-sharing CuO₆ octahedra and separated by Ge ions. Because of this lattice structure, the magnetic properties of CuGeO₃ are shown to be well described by the 1D antiferromagnetic Heisenberg model with S = 1/2 spins. Moreover, this material undergoes Spin-Peierls transition at a temperature T_SP ≈ 15 K. Spin-Peierls transition is the magnetic transition with the lattice distortion, where the period of the lattice has just doubled due to the dimerization of neighboring spin couplings. This spin-lattice coupling is similar to the Peierls transition, the charge-lattice coupling in 1-D metals.
The crystal structure of Ca_2+xY_2-xCu₅O₁₀ consists of 1-D Cu-O chains formed by the edge-sharing CuO₄ squares. The end-material Ca₂Y₂Cu₅O₁₀ without holes shows an antiferromagnetic ordering of the Cu2+ moment below 29.5 K with ferromagnetic coupling along the chain. This 1-D Cu-O chains have two kinds of magnetic interactions, Cu-O-O-Cu superexchange (J₁) with almost 180° angle and Cu-O-Cu superexchange (J₂) with nearly 90° angle. Since J₁ and J₂ have opposite sign, i.e., antiferromagnetic and ferromagnetic, Cu spins should show the spin-frustration. Because of this feature, it is expected that the magnetic ground state crosses over from a Néel state to a new type of short-range-ordered state as a function of hole concentration. The change in magnetic superexchange in this higher hole concentration regime is supposed to be an important part of the understanding of the CuO₂ planes in cuprate.
In this study, the author reports the magnetic properties of CuGe_1-xAl_xO₃ (0.0≦x≦0.4), Cu_1-xCoxGeO₃ (0.0≦x≦0.12), Ca₂Y_2-xSr_xCu₅O₁₀ (0.0≦x≦0.5) and Ca₂Y_2-xMg_xCu₅O₁₀ (0.0≦x≦0.4) compounds.

<3> Chemical and thermoelectric properties of transition-metal oxides with such as double-perovskite and low-dimensional structure

Double-perovskite compounds exhibit various physical properties due to strongly-correlated electron behaviors such as giant magnetoresistance, negative thermal expansion, multiferroics and superconductivity. Sr₂YRuO₆ compound with double-perovskite structure is reported to show high Seebeck coefficient (S = -475 μV/K) and low thermal conductivity (κ = 0.4 W/mK), which are suitable properties for a thermoelectric material. Its electrical conductivity (σ = 0.045 S/cm) is not enough, however, to increase the dimensionless figure of merit (ZT = S²σT/κ), which is related to the efficiency of a thermoelectric device for electricity generation. In this paper, the author has studied chemical and thermoelectric properties of Sr₂YRuO₆ compounds to improve their electrical conductivity leading to higher value of the thermoelectric figure of merit.
Polycrystalline samples of Sr_2-xLa_xYRuO₆ and S₂Y_1-xPr_xRuO₆ were prepared by solid-state reaction (SSR) at 1523-1673 K. In order to increase relative density of the material, obtained samples were exposed to pulsed electric-current pressure sintering (PECPS). Samples were evaluated by X-ray diffraction (XRD), magnetic susceptibility, and thermoelectric property measurements. From XRD measurements, single phases were obtained by both methods of SSR (0 ≤ x ≤ 0.3) and PECPS (0 ≤ x ≤ 0.2). Magnetic susceptibility measurements indicated that the valence of Ru decreases with increasing La or Pr content x. Electrical conductivity was found to be enhanced by the elemental substitution and the densification, leading to maximum value (10.25 S/cm) for Sr_1.7La_0.3YRuO₆ synthesized by PECPS. The values of Seebeck coefficient for all the sample were negative and the absolute value |S| was found to be maximum (-473.57 μV/K) for Sr₂YRuO₆ obtained by SSR. Thermal conductivity measurements showed 0.41-2.85 W/mK for all samples. The calculated ZT was 1.58×〖10〗^^(-2) at most for Sr₂Y_0.8Pr_0.2RuO₆ by SSR method.

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