Polymorphism of monatomic iodine

TL;DR

This study reveals new high-pressure phases of atomic iodine, including a novel Pm-3n structure with distinct atomic sites, expanding understanding of phase transitions in halogens under extreme conditions.

Contribution

The paper reports the discovery of a new Pm-3n phase of atomic iodine at high pressure, combining experimental and theoretical methods, which differs from the traditional I4/mmm structure.

Findings

Identification of a new Pm-3n iodine phase at 48-51 GPa

Observation of face-centered cubic iodine at lower pressures

Distinct atomic arrangements in high-pressure iodine phases

Abstract

We applied synchrotron single-crystal X-ray diffraction in a diamond anvil cell at 48-51 GPa and first-principles theoretical calculations to study the crystal structure of solid atomic iodine at high pressure. We report the synthesis of two phases of atomic iodine at 48-51 GPa via laser heating of I-N2 mixtures. Unlike the familiar monatomic I4/mmm structure, which consists of crystallographically equivalent atoms, a new Pm-3n structure is of inclusion type, featuring two distinct kinds of atoms: a central detached one and peripheral ones forming the linear chains. Moreover, we observe crystallization of the familiar high-pressure face centered cubic (fcc) structure, albeit at much lower pressures compared to cold compressed iodine. The discovery of Pm-3n structure in iodine marks an important step in understanding of the pressure induced phase transition sequence in halogens.