Two intermediate incommensurate phases in the molecular dissociation process of solid iodine under high pressure

A high-pressure phase of solid iodine is identified in the pressure range of 16-23 GPa at room temperature by reanalyzing the previous powder x-ray diffraction data. The phase VI has a four-dimensional (4D) incommensurately modulated structure, in which I-2 molecules and zigzag chains of three iodine atoms coexist. This structural model can successfully explain the Raman scattering data previously reported. Furthermore, our method clarifies that the structure of phase V, which was once analyzed as a 4D incommensurately modulated structure, can be better described by a five-dimensional incommensurately modulated tetragonal lattice wherein two modulation waves run in orthogonal directions. Our findings show that the pressure-induced molecular dissociation proceeds in multiple stages that are more complex than previously thought. In addition, it has become necessary to describe the molecular dissociation pressure separately for the starting pressure P-s of 16 GPa and the finishing pressure P-f of 29 GPa. This transition process has finally resolved the contradiction with the Mossbauer experiment.