Structural aspects of solid iodine associated with metallization and molecular dissociation under high pressure

The x-ray structure analysis of solid iodine has been made at room temperature and at various pressures up to 30 GPa, across the previously discovered phase-transition point $P_t=21\mathrm{}$ GPA. In the low-pressure phase compression causes the nearest-neighbor intermolecular distance to approach the intramolecular one and the angle between the adjacent molecules to approach 90°. This structural change implies a significant increase of band overlapping which results in the metallization, well known in this system. The culmination of such structural trends in the low-pressure phase leads to a transition, at $P_t$, to a body-centered orthorhombic Bravais lattice formed upon dissociation into monatomic iodine. Atomic displacements in the low-pressure phase can be produced by dimerization, which is described by two displacement waves in the higher-symmetry high-pressure phase. A qualitative discussion on this transition has been made based on a Landau theory constructed by such a decomposition of displacements.