Synthesis of ternary compound in H-S-Se system at high pressures

The chemical reaction products of elemental sulfur (S), selenium (Se), and molecular hydrogen (H₂) at high pressures and room temperature are probed by Raman spectroscopy. Two known compounds H₂S and H₂Se can be synthesized after laser heating at pressures lower than 1 GPa. Under further compression at room temperature, an H₂S-H₂Se and an H₂S-H₂Se-H₂ van der Waals compounds are synthesized at 4 GPa and 6 GPa, respectively. The later is of guest-host structure and can be identified as (H₂S)_x(H₂Se)_(2-x)H₂. It can be maintained up to 37 GPa at least, and the stability of its H₂Se molecules is extended:the H-Se stretching mode can be detected at least to 36 GPa but disappears at 22 GPa in (H₂Se)₂H₂. The pressure dependence of S-H and Se-H stretching modes of this ternary compound is in line with that of (H₂S)₂H₂ and (H₂Se)₂H₂, respectively. However, its hydrogen subsystem only shows the relevance to (H₂S)₂H₂, indicating that this ternary compound can be viewed as H₂Se-replaced partial H₂S of (H₂S)₂H₂.