Anomalous bond-length behaviors of solid halogens under pressure

The three halogen solids (Cl₂, Br₂, and I₂) have the isostructural diatomic molecular phase I with a space group of Cmca at ambient pressure. At high pressure, they all go through an intermediate phase V with incommensurate structures before eventually dissociating into the monatomic phase Ⅱ. However, a new structural transition between phase I and V with anomalous bond-length behavior was observed in bromine under pressure, which, so far, has not been confirmed in iodine and chlorine. Here, we perform first-principles calculations for iodine and chlorine. The new structural transition was predicted to be common to all three halogens under pressure. The transition pressures might be systematically underestimated by the imperfect van der Waals correction method, but they follow the order Cl₂ > Br₂ > I₂, which is consistent with other pressure-induced structural transitions such as metallization and the molecular-to-monatomic transition.