  Hydrogen sulfide (H2S) is a prototype molecular system and a sister molecule
of water. The phase diagram of solid H2S at high pressures remains largely
unexplored arising from the challenges in dealing with the looser S-H bond and
larger atomic core difference between H and S. Metallization is yet achieved
for water ice, but it was established for H2S above 96 GPa. However, the
metallic structure of H2S remains elusive, greatly impeding the understanding
of its metallicity and the potential superconductivity. We have performed an
extensive structural study on solid H2S under high pressures through unbiased
first-principles structure predictions based on swarm intelligence. Besides the
findings of best-known candidate structures for nonmetallic phases IV and V, we
are able to establish stable metallic structures violating an earlier proposal
of elemental decomposition into sulfur and hydrogen [PRL 85, 1254 (2000)]. Our
study unraveled a superconductive potential of metallic H2S with an estimated
maximal transition temperature of ~ 80 K at 160 GPa, higher than those
predicted for most archetypal hydrogen-containing compounds (e.g., SiH4 and
GeH4, etc).
