Hydrogen is the simplest bipolar element and its valence state can be controlled from +1 to ?1. We synthesized the 1111-type iron arsenides CaFeAsH and LnFeAsO1?xHx (Ln = lanthanide; 0 ? x ? 0.5) with the ZrCuSiAs type structure by a high-pressure synthesis method. The position and valence state of the substituted H were determined by neutron diffraction and density functional theory calculations. The close similarity in the structural and electrical properties of CaFeAsH and CaFeAsF indicated the formation of the hydride ion (H?), which is isovalent with the fluoride ion (F?), in the 1111-type iron arsenides. When some of the O2? ions in LnFeAsO are replaced by H?, superconductivity is induced by electron doping to the FeAs-layer to maintain charge neutrality. Since the substitution limit of hydrogen in LnFeAsO (x ≈ 0.5) is much higher than that of fluorine (x ≈ 0.2), the hydrogen substitution technique provides an effective pathway for high-density electron-doping, making it possible to draw the complete electronic phase diagram of LnFeAsO. The x–T diagrams of LnFeAsO1?xHx (Ln = La, Ce, Sm, Gd) have a wide superconducting (SC) region spanning the range x = 0.04–0.4, which is far from the parent antiferromagnetic region near x = 0.0. For LaFeAsO1?xHx, another SC dome region was found in the range x = ～0.2 to ～0.5 with a maximum Tc = 36 K, in addition to a conventional SC dome located at x ～ 0.08 with maximum Tc = 29 K. Density functional theory calculations performed for LaFeAsO1?xHx indicated that the newly observed Tc is correlated with the appearance of degeneration of the Fe 3d bands (dxy, dyz and dzx), which is caused not only by regularization of the tetrahedral shape of FeAs4 due to chemical pressure effects but also by selective band occupation with doped electrons. In this article, we review the recent progress of superconductivity in 1111-type iron (oxy)arsenides and related compounds induced by hydrogen anion substitution.
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