Since receiving a short email reporting the superconductor reaching 200 K, we have been working on the reproducing test and the crystal structural determination of the system. We have applied our high-pressure developments for the possible experiments on metallic hydrogen to these experiments. Here we report the progress in last 3+ years including the latest results and our prospective view.

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Hydrogen sulfide H₂S had been interested in its structural behaviors at low temperature and high pressure as a similar material with H₂O ice and as a hydrogen bonding system until 2004. Many solid phases were found and their crystal structures, molecular motions, and hydrogen bonding state were studied by nuclear magnetic resonance, raman, infrared, x-ray, and neutron experiments. Research on hydrogen sulfide became very active again after the discovery of superconductivity at 200 GPa in 2014. The authors think that the crystal structures of H₂S previously reported should be re-examined by using recent structure-analysis techniques with density functional theory calculations. In this article, we will summarize recent structural aspects about some phases of hydrogen sulfide.

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In this article, recent advances in the experimental searching are reviewed. To date, there have been several tens of theoretical papers that predict the superconductivity of hydrides. On the other hand, there have been a few publications about experimental observations of the superconductivity despite the huge efforts in exploration. In this review, we introduce the ideas on how to approach to high critical temperature by hydrides, and the current achievements by experiments are summarized.

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The idea of looking for metallic states in hydrides rather than in pure hydrogen has remarkably increased the options for attaining the high-temperature superconductivity predicted by Ashcroft in 1968. This article presents a few viewpoints which might serve as a practical guide to searching new metallic hydrides which possess high superconducting transition temperatures. According to the Goldhammer-Herzfeld criterion, it is important to pay attention to unstable compounds at one atmospheric pressure because they can be good metals at high pressures. It is also helpful to make use of possible structural similarities of hydrides between the diagonally adjacent elements in the periodic table in order to control insulator-to-metal transition. The effects of anharmonic proton zero-point energy on stable structures are also discussed by applying the self-consistent harmonic approximation to solid hydrogen.

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Novel complex hydrides have been synthesized under high pressure and high temperature based on first-principles prediction. Hydrogen is extremely reactive at high pressure above 1 GPa, thus novel hydrogenation reactions of metals are expected to proceed to form novel hydrides. Theoretically-predicted iron-containing complex hydrides were synthesized at such high-pressure region. The reaction conditions were searched with the aid of in situ synchrotron radiation x-ray diffraction measurements. We demonstrate that the combination of high-pressure technique, theoretical prediction, and in situ synchrotron x-ray diffraction measurement is effective for obtaining novel hydrides.

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