High-pressure Nucleation of Low-Density Polymorphs*.

Area of Science:

• Crystallography
• Materials Science
• Physical Chemistry

Background:

• Polymorphism describes a material's ability to exist in multiple crystal structures.
• Understanding pressure-induced phase transitions is crucial for materials science.
• Bis-3-nitrophenyl disulphide exhibits complex polymorphic behavior under varying conditions.

Purpose of the Study:

• To investigate the formation and properties of new polymorphs of bis-3-nitrophenyl disulphide under high pressure.
• To elucidate the relationship between pressure, entropy, kinetics, and polymorph density.
• To compare pressure-induced crystallization with mechanochemical methods.

Main Methods:

• Crystallization of bis-3-nitrophenyl disulphide at pressures above 0.3 GPa.
• Density measurements of ambient and high-pressure polymorphs.
• Analysis of nucleation, kinetics, and thermodynamic factors (entropy, chemical potential).
• Investigation of pressure-dependent viscosity and Ostwald's rule of stages.

Main Results:

• New polymorphs (β and γ) of bis-3-nitrophenyl disulphide were formed above 0.3 GPa.
• These high-pressure polymorphs are less dense than the ambient polymorph (α).
• This counterintuitive density is attributed to high-entropy nucleation and kinetic crystallization.
• Increased viscosity under pressure favors kinetic polymorphs, aligning with Ostwald's rule.

Conclusions:

• Pressure can induce the formation of less dense polymorphs through kinetic control.
• The interplay of entropy, temperature, and pressure-dependent viscosity governs polymorph selection.
• Pressure-induced crystallization contrasts with mechanochemical methods, which tend to yield denser polymorphs.