Synthesis and stability of hydrogen selenide compounds at high pressure

The observation of high-temperature superconductivity in hydride sulfide (H₂S) at high pressures has generated considerable interest in compressed hydrogen-rich compounds. High-pressure hydrogen selenide (H₂Se) has also been predicted to be superconducting at high temperatures; however, its behaviour and stability upon compression remains unknown. In this study, we synthesize H₂Se in situ from elemental Se and molecular H₂ at pressures of 0.4 GPa and temperatures of 473 K. On compression at 300 K, we observe the high-pressure solid phase sequence (I-I'-IV) of H₂Se through Raman spectroscopy and x-ray diffraction measurements, before dissociation into its constituent elements. Through the compression of H₂Se in H₂ media, we also observe the formation of a host-guest structure, (H₂Se)₂H₂, which is stable at the same conditions as H₂Se, with respect to decomposition. These measurements show that the behaviour of H₂Se is remarkably similar to that of H₂S and provides further understanding of the hydrogen chalcogenides under pressure.