We discovered iron-based layered superconductors: F-doped LaFePnO (Pn=P, As) in 2006. Several superconductors containing square iron lattice have been reported in 2008. Superconducting transition temperature (T_c) of F-doped LnFeAsO (Ln=Nd, Sm, ...) reaches ~55 K. LaFePnO is a 3d transition-metal (T_M) based oxypnictides: LnT_MPnO. The crystal structure is composed of carrier conducting T_M-Pn layer and insulating Ln-O layer. Undoped LnFePO (Ln=La, Sm) shows T_c ~3-5 K, while undoped LnFeAsO shows no T_c. The undoped LnFeAsO takes a orthorhombic crystallographic phase at temperatures below ~160 K, F-doping suppresses the structural transition from tetragonal to orthorhombic phase, stabilizing the tetragonal lattice of LnFeAsO. F-doped LnFeAsO with tetragonal symmetry exhibits T_c.

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  In this article, availability of high-pressure technique to synthesize oxygen-deficient LnFeAsO_1-y (Ln: lanthanide elements) and (Ba, K)Fe₂As₂ samples is demonstrated. It is found out that the synthesis pressure is an important parameter for synthesizing single-phase samples, in particular for the heavier Ln's, such as Tb and Dy. The lattice parameters systematically decrease with the atomic number of Ln, reflecting the shrinkage of Ln ionic radius. For the lighter Ln's (La, Ce, Pr, Nd), T_c increases monotonously with decreasing the lattice parameters from 26 K for Ln=La to 54 K for Ln=Nd, then stays at the constant value around 53 K for the heavier counterpart (Nd, Sm, Gd, Tb and Dy).

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   In the first half of this article, the new material search for FeX₄ (X=pnictogen, chalcogen) based superconductors is introduced and the new method to grow single crystals for Fe-based superconductors and related materials are also described. Using a high-pressure apparatus, we report the single crystal growth of Fe-based superconductors and related materials (BaFe₂As₂, SrFe₂As₂, (Ba, Na)Fe₂As₂ and α-PbO type FeSe_0.9 superconductor, BaNi₂P₂). Especially, we have grown single crystals of BaNi₂P₂ superconductors from stoichiometric composition solutions for the first time. The temperature dependence of in-plane (r_//) and out-of-plane (r_⊥) resistivity for BaNi₂P₂ crystals were measured, the anisotropy ratio r_⊥/r_// is ~6.3 at 300 K, and sharp superconducting transition was observed at ~2.5 K.

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  Crystal structures of iron based superconductors are introduced. Powder neutron diffraction measurements reveal that their crystal structure and superconductivity are strongly correlated. Superconducting transition temperature (T_c) attends maximum value when FeAs₄-tetrahedrons form a regular shape. An ideal crystal structure for achieving higher T_c is discussed.

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  Electrical resistivity and magnetic susceptibility measurements under high pressure were performed on iron-based superconductor LaFePO and LaFeAsO_1-xF_x system. A steep increase in superconducting transition temperature (T_c) was observed for LaFePO and LaFeAsO_1-xF_x. Pressure-induced superconductivity was confirmed in undoped LaFeAsO and SrFe₂As₂ by electrical resistivity measurements under high pressure. X-ray diffraction measurements were also performed under high pressure up to 10 GPa for LaFePO, LaNiAsO and LaFeAsO_1-xF_x system, where the anisotropic decreases of the lattice constants were observed with applying pressure. Recent advance in the high pressure reserches for iron-based high T_c superconductors are also presented.

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  In this article, we review recent NMR studies of Fe-based superconductors. We also report our ⁷⁵As-NMR and dc resistivity measurement of BaFe₂As₂ under high pressure P. The T-P phase diagram of BaFe₂As₂ determined from resistivity anomalies and the ⁷⁵As-NMR clearly revealed that the antiferromagnetic anomaly is quite robust against P. The NMR measurement under P beyond 3 GPa is needed in order to determine the ground state of BaFe₂As₂ under P. In addition, we report recent development of NMR technique under high P.

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  In this article, theoretical aspects of the newly discovered iron pnictide superconductors are reviewed. First we present the effective model, which turns out to contain all five 3d orbitals. Then we show how the nesting of the disconnected sheets of the Fermi surface can result in multiple modes of spin fluctuations. When one of the spin fluctuation modes dominate, the superconductivity has an s-wave symmetry where the gap changes sign between different sheets of the Fermi surface. On the other hand, when the spin fluctuation modes compete with each other, d-wave pairing becomes competitive or even dominates over s-wave pairing. We explain how the lattice structure may affect the Fermi surface and thus the T_c as well as the competition between different pairing symmetries.

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  This article has been contributed as a memorial of receiving the award from The Japan Society of High Pressure Science and Technology in 2008. Fundamentals and applications of gas hydrates, which are investigated in our research group after 1990s, are integrated. Quite recent topics on the radical behavior trapped in the hydrate cage are also reviewed. It is my great pleasure to be helpful to young researchers in the gas hydrate fields.

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  Physical properties of liquid iron-alloy under high pressure control the planetary core formation, evolution and its dynamics. In this review paper, high pressure behaviors of some physical properties (viscosity, wetting property and interfacial tension) are discussed and applied to the planetary core. The viscosities of Fe-S and Fe-C liquids measured using X-ray radiography falling sphere method up to 16 GPa show low viscosity values (~10 mPa-s) and the activation volume of viscous flow is also very small (~1.5 cm³/mol). The influence of light elements on the viscosity and the activation volume has only a minor contribution. Dihedral angle, i.e. wetting property of Fe-S-O liquid among mantle minerals is mainly controlled by an interfacial energy of liquid iron-alloy and not by that of solid phase. The effect of light elements on interfacial tension of Fe-S and Fe-P liquids measured using sessile drop method depends on element species. Sulphur corresponds to a surface-active element. The trend observed at high pressure is quite consistent with those at ambient pressure.

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