Compression Rate-Dependent Crystallization of Pyridine

The high-pressure polymorphism of pyridine has attracted great attention. Herein, the crystallization process of pyridine at different compression rates was investigated in detail. When the compression time was <= 1 s, pyridine crystallized into phase III, while phase I occurred when the compression time was >= 10 s, although both phases were compressed from 0.30 to similar to 1.20 GPa. In situ time-resolved ruby fluorescence and optical micrographs of the pyridine compression process revealed that the compression rate affected the initial crystallization pressure and resulted in different crystalline phases. Phase I slowly transformed to phase III at 1.55 GPa, while the reverse transformation occurred rapidly upon decompression to 1.0 GPa. In situ Raman spectroscopy and synchrotron X-ray diffraction results demonstrated that phase III did not undergo any further phase transition up to 17 GPa. Our results proved that the compression rate could selectively control the nucleation pathway to achieve different final products even when the final pressure was the same. The findings will enable a better understanding of the high-pressure crystallization kinetics of the materials.