Molecular Mechanism for the Cross-Nucleation between Polymorphs

摘要

Understanding and controlling in which structure (or polymorph) a molecule crystallizes are complex and long-standing issues.In many cases,a process yields crystals of more than one polymorph at the same time,making the control of polymorphism even more challenging.Several mechanisms have been proposed to account for this phenomenon,known as concomitant polymorphism.It has been attributed either to competing processes of homogeneous nu-cleation of different polymorphs or to solvent-mediated conversion of one polymorph into another or,more recently,to the heterogeneous nucleation (or cross-nucleation) of one polymorph on another.In this work,we use molecular dynamics simulations to study the early stages of crystallization in a supercooled liquid of spherical particles.We observe the onset of concomitant polymorphism and demonstrate that this phenomenon results from the cross-nucleation of a metastable polymorph on the stable polymorph.We also show that cross-nucleation is selective since it only takes place between polymorphs of almost equivalent free energy.Our simulations provide detailed insights into the molecular mechanism underlying concomitant polymorphism and cross-nucleation.