Various transparent nano-ceramics have been synthesized using direct conversion from bulk glass starting materials under pressures higher than ～10 GPa and at high temperatures in Kawai-type Multi-anvil Apparatus (KMA). Rapid nucleation followed by slow crystal/grain growth is the key to synthesize such nano-ceramics at the ultra-high pressure regime. Thus synthesized novel ceramics have interesting features, such as higher hardness and lower thermal conductivity relative to those of the corresponding single crystals, in addition to their high optical transparencies, which may be useful for some scientific and industrial applications.

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Nanopolycrystalline stishovite was fabricated under high pressure and temperature using Kawai-type apparatus. This material is the hardest oxide as well as one of the toughest ceramics with fracture toughness of about 12 MPa m^1/2. We performed X-ray absorption spectroscopy using synchrotron radiation, bending tests using micro-cantilever specimens fabricated by focused ion beam technique, and first-principles calculations for this material. Using the obtained results, we demonstrated that this material is toughened by transformation toughening mechanism by fracture-induced amorphization of stishovite. This toughening mechanism can be a platform to produce new structural materials.

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Synthesis of cubic boron nitride (cBN) and aluminum nitride (AlN) single crystals and rare-earth (RE) element doping were carried out by using a belt-type high-pressure (HP) apparatus. Typical emission spectra originated by doped RE, such as Ce and Eu, were observed. Ab-initio theoretical study and transmission electron spectroscopy analyses were carried out to elucidate a capability of cBN and AlN system with large size mismatch dopant system.

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Recent advances in high-pressure synthesis and theoretical investigation of novel transition metal oxide catalysts for oxygen evolution/reduction reactions (OER/ORRs) are reviewed. Valence dependence of OER catalysis for perovskite oxides are demonstrated by comparative study of iron perovskite oxides synthesized under high pressure. Structure effects on OER catalytic activity are investigated in manganese oxides with simple ABO₃-type and quadruple AA′₃B₄O₁₂-type structures. In addition to intrinsic high ORR activities, electrochemical experiments display that OER activities of quadruple perovskites AMn₇O₁₂ (A=Ca, La) are superior to those of simple perovskite AMnO₃ counterparts, leading to OER/ORR bifunctional catalysis. Theoretical calculations propose a OER mechanism for quadruple manganese perovskite oxide, in which adsorption sites bridging between A′- and B-site Mn atoms play a crucial role in lowering OER overpotentials.

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In this article, our recent researches on the crystal structure of the perovskite-type oxyfluorides were reviewed. Parts of these compounds were synthesized using the cubic-multi-anvil type high-pressure apparatus. In cubic oxyfluorides, the displacement from ideal positions of an anion or a cation was observed and the ion species displaced depended on the tolerance factor. The oxyfluorides containing Pb and/or Mn ions had a tetragonal structure and PbMnO₂F containing both ions exhibit very high tetragonality (c/a=1.10). In addition, the solid solutions between a perovskite-type oxide and an oxyfluoride or a fluoride were synthesized using high-pressure apparatus. As the result, it was revealed that one of these solid solutions, (1−x)BaTiO₃-xKTiO₂F represented unique structures and interesting dielectric properties.

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Recent progress on perovskite-type oxides with unusually high-valence Fe⁴⁺ and Co⁴⁺ ions, especially for the latter system, is reviewed with showing magnetic phase diagrams. All the compounds are obtained by high-pressure oxygen annealing for the oxygen-deficient perovskites in the form of polycrystalline or single-crystalline samples prepared at ambient pressure. While cubic perovskite-type SrFeO₃ has been found to show various types of novel helimagnetic phases, cubic perovskite-type SrCoO₃ known as a room-temperature ferromagnet has been revealed to show helimagnetic ordering either by Ba substitution or Ca substitution for Sr. These systems provide rich magnetic phase diagrams and anomalous cross-correlation effects such as a giant inverse magnetovolume effect, which are inherent to the strong p-d hybridization generating itinerant oxygen ligand holes.

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Due to a small energy gap between HOMO and LUMO, the single-component molecular crystals based on metal dithiolene complexes are attracting great interest. Metallic single-component molecular crystals are difficult to obtain, compared with semiconducting or insulating ones. Here we performed the high pressure resistivity measurements on semiconducting single-component π molecular crystals, and found that some of them turned to metal, superconductor, and Dirac material under high pressure.

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