Polymorphism of monatomic iodine

We applied synchrotron x-ray diffraction in a diamond-anvil cell at 48--51 GPa, alongside first-principles theoretical calculations, to study the crystal structure of solid atomic iodine at high pressure. We report the synthesis of two phases of atomic iodine at 48--51 GPa via laser heating of iodine (I) in nitrogen $({\mathrm{N}}₂)$ or neon (Ne) media. Unlike the familiar monatomic $I4/mmm$ structure, which consists of crystallographically equivalent atoms, a previously unobserved $Pm\overline{3}n$ structure determined by single-crystal and powder x-ray diffraction is of an inclusion type, featuring two distinct types of atoms: a central, detached atom and peripheral atoms that form linear chains. Additionally, we observe crystallization of the familiar high-pressure fcc structure, albeit at much lower pressures compared to cold-compressed iodine. The discovery of the $Pm\overline{3}n$ structure in iodine marks an important step in understanding the pressure-induced phase transition sequence in halogens.