The microscopic transition process from high-density to low-density amorphous state of SnI₄

Area of Science:

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

Background:

• Molecular crystalline SnI4 transforms into a high-density amorphous (HDA) state (Am-I) under pressure via molecular dissociation.
• Understanding the reverse transition from Am-I to the low-density amorphous (LDA) state (Am-II) is key to characterizing amorphous materials.

Purpose of the Study:

• To investigate the reverse amorphization process of SnI4 from the HDA state (Am-I) to the LDA state (Am-II).
• To identify the structural and molecular changes occurring during decompression and the HDA-LDA transition.

Main Methods:

• In situ angle-dispersive synchrotron X-ray diffraction measurements using a diamond anvil cell.
• Density estimation derived from structural data.
• Molecular dynamics simulations combined with reverse Monte Carlo fitting.

Main Results:

• The HDA (Am-I) to LDA (Am-II) transition occurs abruptly between 3.3 and 3.0 GPa upon decompression.
• Gradual molecular reassociation within the Am-I state is observed between 18 and 14 GPa.
• The Am-I state is differentiated into high-pressure (isolated Sn atoms) and low-pressure (deformed molecules with I2 bonds) substates.
• A local C3 molecular symmetry in Am-I precedes the transition to Am-II, where original Td symmetry is recovered.

Conclusions:

• The reverse amorphization of SnI4 involves distinct stages of molecular reassociation and local symmetry changes.
• The observed local symmetry changes correlate with the global density changes, similar to liquid-liquid transitions.
• This study provides insights into the complex structural evolution of amorphous materials under pressure and decompression.