Structural evolution of iodine on approach to the monatomic state

TL;DR

This study investigates the high-pressure structural evolution of solid iodine, revealing a sequence of phase transitions from molecular to monatomic states through experimental and theoretical methods up to 36 GPa.

Contribution

It provides new insights into iodine's phase transitions under pressure, including the identification of a previously unreported incommensurate phase and the detailed pathway to a monatomic state.

Findings

Molecular phase transforms to Cmc21 at 16 GPa

Formation of incommensurate Fmmm(00γ)s00 structure at 20 GPa

Transition to monatomic Immm phase at higher pressures

Abstract

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.