Quadruple perovskite oxides AA'₃B₄O₁₂, which can be synthesized under high pressures above several GPa, demonstrate multifunctional properties derived from their structural and electronic features. In this article, we review the recent progress in the intriguing functions of the quadruple perovskite series. RCu₃Fe₄O₁₂ (R = La, Pr, Nd, Sm, Gd) exhibit robust thermal-energy storage properties. CeMn₃Cr₄O₁₂ displays metamagnetic behavior which is attributed to the 4f magnetic moments of Ce³⁺ ions. thermal storage. Systematic comparisons between simple and quadruple perovskite oxides demonstrate that structural feature of the quadruple perovskite serves to enhance the catalytic activity for oxygen evolution reaction.

This article reviews developments of skutterudite-type thermoelectric (TE) materials by high pressure synthesis using multi-anvil apparatus. TE materials have been utilized in power generation devices because they can convert thermal energy directly into electrical energy. TE power generation using waste heat has attracted much attention as one of the new energy resources. CoSb₃-based skutterudite compounds have attracted considerable attention as one of the best candidates of TE materials. We focused on reduction of lattice thermal conductivity in skutterudite compounds to improve the TE performance. We demonstrate the possibility of developing high-performance TE materials by preparing new compounds using high-pressure synthesis and pressure-induced reactions.

The development of new superconductors (SCs) is important not only for their application but also for their potential to contribute to the progress of solid-state physics and materials science. In this article, we describe an approach to efficiently search for new SCs utilizing a combinatorial chemistry method. The method consists of preparation of bulk samples containing numerous various compounds and an effective screening method to detect new SCs in the bulk sample. We will show the effectiveness of the bulk sample preparation using high-pressure synthesis method and examples of new SCs discovered by this combinatorial method.

The recent development of high-pressure experimental techniques combined with advanced characterization and analytical methods allows us to reveal the crystal chemistry of the transition metal-nitrogen binary compounds up to the megabar pressure regime. Novel early transition metal-nitrogen compounds, VN₂, Nb₂N₃, and MoC-type WN_0.6, have been synthesized by using a laser-heated diamond-anvil cell or DIA-type high-pressure apparatus. In addition to conventional high-pressure in-situ X-ray diffraction measurements, the low-temperature thermal expansion behavior and the valance state of transition metal ions of these newly synthesized compounds were revealed. In this article, the details of our experiments, and recent progress in high-pressure synthesis and crystal chemistry of transition metal nitrogen binary compounds are reviewed.

In this article, the author discusses what high-pressure synthesis do for new functional materials search, based on his experience on synthesis of silicides, Si clathrates, and filled skutterudites. The materials synthesized at high pressures are denser than the materials synthesized at the ambient pressure. The author used high-pressure synthesis to (1) synthesize the compounds with crystal structures predictable from structural sequences in pressure-induced structural phase transitions and elemental substitutions in similar materials, (2) synthesize the intercalation-type host-guest compounds those have the similar crystal structure to existing Zintl-phase ones, (3) synthesize Zintl-phase tetrides that have more anion contents than those synthesized at ambient pressure, and (4) increase solid solution limits in compounds whose volume is increased by elemental substitutions.

Over the past six decades, I have maintained research collaborations with Japanese scientists while pursuing an academic career focusing on scientific investigations of the physical properties of minerals at high pressures and temperatures. I first visited Japan in 1971; in the ensuing half century, I have paid another dozen visits to research laboratories in Tokyo, Kyoto, Osaka, Nagoya, Tsukuba, Sendai, Sapporo, Matsuyama, Hiroshima, and Misasa. The objective of this paper is to relate this history.