Enhancement of T c in the atomic phase of iodine-doped hydrogen at high pressures

The high-pressure structures and superconductivity of iodine-doped hydrogen have been studied by ab initio calculations. Above 100 GPa, we discover a stable phase with Pnma symmetry in the H2I stoichiometry that consists of a monatomic iodine tube trapping hydrogen molecular units. Interestingly, H2 molecular units dissociate and form a novel atomic phase with R3̄m symmetry at 246 GPa. Further electron-phonon coupling calculations predict the critical temperature of superconductivity T(c) to be 3.8 K for the Pnma phase and 33 K for the R3̄m phase at 240 GPa. Significantly, the T(c) of the R3̄m phase is enhanced approximately 8 times that of the Pnma phase, which is mainly attributed to the reason that H2 molecules are broken exhibiting an atomic character in the R3̄m phase.