Structures and novel superconductivity of hydrogen-rich compounds under high pressures

Metallic hydrogen can be realized theoretically at high pressure, which suggests that it will be a room-temperature superconductor due to the high vibrational frequencies of hydrogen atoms. However, the metallic state of hydrogen is not observed in experiment at up to 388 GPa. Scientists have been exploring various new ways to achieve hydrogen metallization. Hydrogen-rich compounds can be metallized at much lower pressures because of chemical pre-compression. Moreover, because such materials are dominated by hydrogen atoms, some novel properties can be found after metallization, such as high Tc superconductivity. Therefore, hydrogen-rich compounds are potential high-temperature superconductors, and this method is also believed to be an effective way to metalize hydrogen, which has aroused significant interest in lots of fields, such as physics, material science, etc. In a word, hydrogen-rich compounds are expected to become a new member of superconductor family:hydrogen-based superconductor. Very recently, the theoretical prediction and the successful experimental discovery of high-temperature superconductivity at 200 K in a sulfur hydride compound at high pressure have set a record, which inspired further efforts to study the superconductivity of hydrogen-rich compounds. The present review focuses on crystal structures, stabilities, interaction between atoms, metallization, and superconductivity of several typical hydrogen-rich compounds at high pressures. Furthermore, higher Tc superconductors can be expected to be found in hydrogen-rich compounds in the future.