Structural evolution of iodine on approach to the monatomic state

We applied single-crystal x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to 36 GPa and first-principles theoretical calculations to study the molecular dissociation of solid iodine at high pressure. Unlike previously reported, we find that the familiar Cmce molecular phase transforms to a Cmc21 molecular structure at 16 GPa, and then to an incommensurate polymeric Fmmm(00 & gamma; )s00 structure, which can be viewed as a mixture of molecular I2 and zigzag chains of three iodine atoms, at 20 GPa. The available data are consistent with metallization of iodine at the boundary between the commensurate Cmc21 structure and the incommensurate Fmmm(00 & gamma; )s00 structure at 20 GPa.