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