A.V. Mikheyenkova,b,c aInstitute for High Pressure Physics, Russian Academy of Sciences, Troitsk, Moscow, Russian Federation bMoscow Institute of Physics and Technology (National Research University), Institutskii per. 9, Dolgoprudny, Moscow Region, 141701, Russian Federation cNational Research Center "Kurchatov Institute", Institute for High Energy Physics, pl. Nauki 1, Protvino, Moscow Region, 142281, Russian Federation
In the last two or three years, the significant advances in the study of hydrogen and its compounds under extreme conditions (ultrahigh pressures over a wide temperature range) have notably improved the hydrogen phase diagram, provided a breakthrough in understanding hydrides under pressure (as exemplified by the discovery of high-temperature superconductivity in hydrogen sulfide), and, finally, enabled 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 theoretical and numerical work it stimulates.