Prof. Shin-ichi Ohkoshi has long been interested in phase transition phenomena in solid materials. Using his fundamental knowledge of chemistry, including chemical thermodynamics, magneto-chemistry, photochemistry, and coordination chemistry, and his innovative design concepts, he has realized groundbreaking phenomena and materials such as new ferromagnetic metal complexes, high-performance magnetic oxides, high-frequency millimeter-wave absorbers, photoinduced phase transition metal oxides, and heat-storage ceramics. He is a global leader and pioneer in the field of materials science. His discoveries and syntheses have made significant contributions to materials science. Below his major achievements are summarized.
1. Ferromagnetic phase transition metal complexes that correlate light and electromagnetic waves
In an effort to develop light-induced phase-transitional and photomagnetic materials, Prof. Ohkoshi synthesized several hundred types of zero-dimensional high-spin clusters to one-, two-, and three-dimensional structure magnetic materials in cyano-bridged magnetic metal complexes. Specifically, he is an international trailblazer in light-induced spin-crossover ferromagnets in Fe2[Nb(CN)8] (4-pyridinealdoxime)8 complexes. He devised the world's first synthesis of a chiral photomagnet. Additionally, he observed a magneto-optical phenomenon called "ninety-degree optical switching of the polarization plane of second harmonic light" and light-induced magnetic pole reversal phenomenon via light-induced magnetic pole reversals (N and S). He has developed Co[W(CN)8] materials, which are a class of octacyano-metal complexes that exhibit the highest magnetic phase-transition temperature and coercivity known to date. Furthermore, his studies on phonon observations by terahertz light have elucidated phenomena of low-frequency oscillations of Cs+ ions in Prussian blue frameworks and a Cs-detection method by terahertz light.
2. Ferromagnetic phase transition metal complexes that respond to heat and chemical stimuli
Prof. Ohkoshi demonstrated both theoretically and experimentally that an ideal combination of ferromagnetism and ferrimagnetism can be achieved in Prussian blue analogs based on material design using computational methods for statistical thermodynamics in the solid state and his own computer program. He was the first to discover a magnetic material with a double pole-reversal phenomenon (two compensation points) due to a temperature change. It was commonly believed that only a single compensation point could exist in magnets. Therefore, he made a revolutionary discovery that challenged the conventional belief. He also synthesized magnets with interesting magnetic properties by integrating between five- to eight-coordination cyanide complexes to synthesize a magnet with a variety of magnetic structures. One example is an alcohol vapor sensitive magnet. A humidity-responsive magnet (a moisture-sensitive magnet) is the world's first example of a magnet that responds to humidity to alter the magnetic characteristics.
3. Ferroelectric-ferromagnetic metal complexes and ionic-conductive-ferromagnetic metal complexes
Prof. Ohkoshi induced a structural phase-transition by applying an external electric field to a Co[W(CN)8](pyrimidine) complex, which displays an electric-charge-transfer phase transition. In addition, he realized ferroelectric-ferromagnetic properties in metal complexes for the first time in the history. He worked on electrochemical processes to produce transparent color magnetic films and synthesize a magnetic complex with pyroelectricity and piezoelectricity. He successfully observed magnetization-induced second-harmonic generation (MSHG). He discovered a ferromagnet using a V[Cr(CN)6] series that shows superionic conductivity at room temperature to realize the first observation of spinionics, which is a phenomenon resulting from the correlation between superionic conductivity and ferromagnetism.
4. Epsilon iron oxide (ε-Fe2O3) that possesses huge coercivity and high-frequency millimeter wave absorption, and millimeter wave material science
Prof. Ohkoshi synthesized the first example of a single-phase epsilon-iron-oxide (ε-Fe2O3). He discovered that this single phase shows the strongest coercivity among all known ferrite magnets at room temperature. Metal-substituted epsilon iron oxide has a coercivity of up to 37 kOe. He elucidated that ε-Fe2O3 is the world's smallest hard ferrite magnet with a ferromagnetic order of up to 7.5 nm, and is continuing research to apply it to high-density magnetic recording tapes. Moreover, he showed that ε-Fe2O3 and metal-substituted epsilon iron oxide have the highest millimeter wave absorption frequency (35 to 222 GHz) among all magnets. Until his discovery, no magnetic material was known to absorb millimeter waves. Currently ε-Fe2O3 is drawing attention as a new material to contribute to the age of big data/IoT. Hence, he is a strong advocate for a new field of study called "millimeter wave material science". Research and development in collaboration with a number of corporations are underway due to the potential applications of ε-Fe2O3 as a millimeter wave absorption material. Moreover, Prof. Ohkoshi has received public acclaim; for example, the British BBC broadcasting introduced to a special exhibition at Science Museum, London based on his work.
5. Lambda trititanium pentoxide (λ-Ti3O5) with light-induced and pressure-induced phase transitions and heat-storage ceramics
Prof. Ohkoshi discovered a new type of titanium-oxide (lambda trititanium pentoxide: λ-Ti3O5) by chemical nanoparticle synthesis. λ-Ti3O5 has metallic properties and displays a light-induced metal-to-semiconductor transition. This is the world's first example of a metal oxide exhibiting a light-induced phase transition at room temperature. In addition, a particular structure of λ-Ti3O5 shows a phase transition with a weak pressure. The phase induced by pressure reverts back to λ-Ti3O5 by absorbing a large heat energy equivalent to 70% of heat fusion of water, and the stored heat energy is kept indefinitely. From this knowledge, he is a firm advocate of a new concept called "heat-storage ceramics", which only release heat energy by external stimuli. Heat-storage ceramics can be utilized to reuse wasted heat energy in factories or to improve heat-energy efficiency in cars. Hence, industries are optimistic for the future contributions of this new material.
Prof. Ohkoshi has conducted revolutionary material development owing to his unique ideas. These remarkable research results have created a paradigm shift in the field of materials science. His contributions have received high academic appraisal worldwide and industries have high hopes to apply his research. For the reasons above, Prof. Ohkoshi's work is highly deserving of the Japan Chemical Society award.