S₈ring content in liquid sulfur determined from x-ray diffraction.

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• The structure of liquid sulfur, particularly the fraction of S8 rings, is crucial for understanding its unique polymerization behavior.
• Established models suggest liquid sulfur primarily consists of S8 rings below the lambda-transition temperature (Tλ).

Purpose of the Study:

• To quantitatively determine the S8 ring content in liquid sulfur across a wide temperature range (-55 to 330 ºC).
• To investigate the structural changes and ring dynamics in liquid sulfur, challenging existing models.
• To correlate the S8 ring fraction with the polymerization mechanism at Tλ.

Main Methods:

• X-ray pair distribution function (PDF) analysis at 4.47 Å to probe S8 ring structure.
• Three distinct methods (scaling, linear combination fit, direct integration) to calculate the S8 ring fraction.
• First-principles molecular dynamics (MD) simulations to investigate dynamic ring distortion and bond angles.

Main Results:

• Approximately one-third of S8 rings dissociate upon melting, forming short chains (average length 5±2 atoms).
• S8 ring fraction varies with temperature: 76±10% in supercooled liquid (-30ºC) and 23±3% in high-temperature melt (330ºC).
• MD calculations confirm dynamic ring distortions and support the presence of short chains in low-temperature liquid sulfur.

Conclusions:

• The study contradicts established models by revealing significant S8 ring dissociation in low-temperature liquid sulfur.
• The presence of short chains is essential for the bond-switching mechanism driving polymerization at Tλ.
• This research provides a revised understanding of liquid sulfur structure and its role in polymerization.