  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
dynamically disordered Fmmm(00{\gamma})s00 structure at 20 GPa, which can be
viewed as a stepwise formation of polymeric zigzag chains of three iodine atoms
following by the formation of the dynamically dissociated, incommensurately
modulated i-Fmmm phase, and the truly monatomic Immm phase at higher pressures.
