Polymorphic selectivity in crystal nucleation.

Area of Science:

• Materials Science
• Physical Chemistry
• Crystallography

Background:

• Polymorphism in glass-forming liquids leads to diverse crystal structures.
• Understanding crystal nucleation rates is crucial for predicting crystallization outcomes.
• ROY and nifedipine (NIF) are richly polymorphic glass-forming liquids used in this study.

Purpose of the Study:

• To measure and compare crystal nucleation rates for different polymorphs in ROY and NIF.
• To investigate the relationship between polymorph properties (energy, density) and nucleation rates.
• To assess the influence of the glass transition temperature (Tg) on nucleation kinetics.

Main Methods:

• Experimental measurement of crystal nucleation rates in supercooled melts of ROY and NIF.
• Analysis of polymorph structures, energies, and densities.
• Kinetic analysis across a temperature range including Tg.

Main Results:

• Specific polymorphs (ROY ON, NIF γ') exhibited significantly faster nucleation rates.
• Faster-nucleating polymorphs showed higher energies and lower densities, resembling the liquid phase.
• Nucleation rates were found to be independent of the glass transition temperature (Tg).
• Nucleation and growth rates were proportional at low temperatures, suggesting a shared kinetic barrier.

Conclusions:

• Classical nucleation theory accurately describes observed rates when crystal growth rate defines the kinetic barrier.
• Quantitative nucleation and growth data allow prediction of crystallization rates and polymorphic outcomes.
• The study highlights that kinetically favored polymorphs, not necessarily the thermodynamically most stable, can dominate crystallization.