Cross-nucleation between ROY polymorphs.

Cross-nucleation between polymorphs is a newly discovered phenomenon important for understanding and controlling crystal polymorphism. It contradicts Ostwald's law of stages and other theories of crystallization in polymorphic systems. We studied the phenomenon in the spontaneous and seeded melt crystallization of 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), currently the most polymorphic system of known structures. We observed extensive and sometimes selective cross-nucleation between ROY polymorphs. Certain polymorphs could not nucleate without the aid of others. The new polymorph was found to be more or less thermodynamically stable than the initial one but to always grow faster than or as fast as the initial one. The temperature and surface characteristics of the seed crystals affected the occurrence of cross-nucleation. Our results show that the pathway of crystallization in polymorphic systems is not determined solely by the initial nucleation, but also by cross-nucleation between polymorphs and the different growth rates of polymorphs. This study identified a new metastable polymorph of ROY, the 10th of the family.