Superconducting High-Pressure Phases Composed of Hydrogen and Iodine.

Evolutionary structure searches predict three new phases of iodine polyhydrides stable under pressure. Insulating P1-H5I, consisting of zigzag chains of (HI)δ+ and H2 δ− molecules, is stable between 30 and 90 GPa. Cmcm-H2I and P6/mmm-H4I are found on the 100, 150, and 200 GPa convex hulls. These two phases are good metals, even at 1 atm, because they consist of monatomic lattices of iodine. At 100 GPa the superconducting transition temperature, Tc, of H2I and H4I is estimated to be 7.8 and 17.5 K, respectively. The increase in Tc relative to elemental iodine results from a larger ωlog from the light mass of hydrogen and an enhanced λ from modes containing H/I and H/H vibrations.