Hydrogen and its compounds under extreme pressure**This review presents an extension of the talk by the authors for the scientific session of the Physical Sciences Division of the Russian Academy of Sciences, “Old and new ideas in phase transition physics,” which was held on December 21, 2016 (see Phys. Usp. 60 948–957 (2017); Usp. Fiz. Nauk 187 1021 (2017)). (Editor’s

Abstract

In the last two or three years, significant advances in the study of hydrogen and its compounds under extreme conditions (ultrahigh pressures over a wide temperature range) have notably changed the hydrogen phase diagram, provided a break-through in understanding hydrides’ behavior under pressure (as exemplified by the discovery of high-temperature superconductivity in hydrogen sulfide), and, finally, enabled achieving cold metallization of hydrogen. The situation prior to the 2010s is reviewed in brief and more recent work is examined in detail. While the primary focus is on experimental research, mention is also made of the theoretical and numerical work it stimulates.