A new phase of solid iodine with different molecular covalent bonds

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Diatomic molecular solids exhibit complex phase transitions under high pressure, including metallization and dissociation.
• Previous models suggested a transition sequence from molecular (phase I) to incommensurate (phase V) to atomic (phase II) phases.
• Unexplained Raman spectral bands (X and Y) were observed in the molecular phase at low pressures, prior to phase V.

Purpose of the Study:

• To investigate the origin of the anomalous low-pressure Raman bands in solid iodine.
• To propose and validate a new phase (phase I") that accounts for experimental observations.
• To enhance the understanding of pressure-induced phenomena in diatomic solids.

Main Methods:

• First-principles calculations were employed to model the behavior of solid iodine.
• X-ray diffraction experiments were conducted to corroborate theoretical findings.
• Analysis of vibrational frequencies and Raman spectra was performed.

Main Results:

• A novel low-pressure phase (phase I") for solid iodine was proposed, characterized by the coexistence of two distinct covalent intramolecular bonds.
• The calculated vibrational modes of phase I' successfully explained the pressure dependence of the observed X and Y Raman bands.
• Experimental x-ray diffraction data confirmed the existence of phase I'.

Conclusions:

• The study successfully explains the previously unresolved Raman spectral features in solid iodine at low pressures.
• The identification of phase I' refines the understanding of pressure-induced phase transitions and molecular dissociation in diatomic solids.
• These findings may offer insights into similar phenomena in other molecular solids, such as solid hydrogen (H2).