Unexpected compound reformation in the dense selenium-hydrogen system

The H₂Se molecule and the van der Waals compound (H₂Se)₂H₂ are both unstable upon room temperature compression, dissociating into their constituent elements above 22 GPa. Through a series of high pressure-high temperature diamond anvil cell experiments, we report the unexpected formation of a novel compound, SeH₂(H₂)₂ at pressures above 94 GPa. X-ray diffraction reveals the metallic sublattice to adopt a tetragonal (I4₁/a m d) structure with density functional theory calculations finding a small distortion due to the orientation of H₂ molecules. The structure comprises of a network of zig-zag H-Se chains with quasi-molecular H₂ molecular units hosted in the prismatic Se interstices. Electrical resistance measurements demonstrate that SeH₂(H₂)₂ is non-metallic up to pressures of 148 GPa. Investigations into the Te-H system up to pressures of 165 GPa and 2000 K yielded no compound formation. The combined results suggest that the high pressure phase behavior of each chalcogen hydride is unique and more complex than previously thought.