Pressure-Induced Synthesis and Properties of an H$_{2}$S–H$_{2}$Se–H$_{2}$ Molecular Alloy

The chalcogens are known to react with one another to form interchalcogens, which exhibit a diverse range of bonding and conductive behavior due to the difference in electronegativity between the group members. Through a series of high-pressure diamond anvil experiments combined with density functional theory calculations, we report the synthesis of an S–Se hydride. At pressures above 4 GPa we observe the formation of a single solid composed of both H$_2$Se and H$_2$S molecular units. Further compression in a hydrogen medium leads to the formation of an alloyed compound (H$_2$S$_x$Se$_{1–x}$)$_2$H$_2$, after which there is a sequence of pressure-induced phase transitions associated with the arrested rotation of molecules. At pressures above 50 GPa, there is a symmetrization of hydrogen bonds concomitantly with a closing band gap and increased reflectivity of the compound, indicative of a transition to a metallic state