Unusual Stoichiometries of Hydrogen and Iodine Under Pressure

Evolutionary structure searches are combined with density functional calculations to examine the most stable stoichiometries and structures of hydrogen rich iodine phases, H$_{\mathrm{n}}$I with n\textgreater 1, under pressure. With respect to decomposition into hydrogen and iodine the first of the stoichiometries, H$_{5}$I, is predicted to become thermodynamically preferred at $\sim$30 GPa and remain stable until H$_{2}$I, consisting of chains of molecular hydrogen within a iodine sublattice, becomes the global thermodynamic minimum at $\sim$90 Gpa. H$_{5}$I consisting of both molecular and atomic hydrogen within H---I---H chains is predicted to remain insulating until $\sim$70 GPa whereas the H$_{2}$I stoichiometry is predicted to become metallic at $\sim$5GPa, well before it becomes thermodynamically stable. A second metallic phase, H4I, present on the convex hull at pressures above 100 GPa is constructed of a graphene-like molecular hydrogen sublattice between layers of a hexagonal iodine sublattice.