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.

A remarkable property of certain glass-forming liquids is that a fast mode of crystal growth is activated near the glass transition temperature Tg and continues in the glassy state. This growth mode, termed GC (glass-crystal), is so fast that it is not limited by molecular diffusion in the bulk liquid. We have studied the GC mode by growing seven polymorphs from the liquid of ROY, currently the top system for the number of coexisting polymorphs of known structures. Some polymorphs did not show GC growth, while others did, with the latter having higher density and more isotropic molecular packing. The polymorphs not showing GC growth grew as compact spherulites at all temperatures; their growth rates near Tg decreased smoothly with falling temperature. The polymorphs showing GC growth changed growth morphologies with temperature, from faceted single crystals near the melting points, to fiber-like crystals near Tg, and to compact spherulites in the GC mode; in the GC mode, they grew at rates 3-4 orders of magnitude faster with activation energies 2-fold smaller than the polymorphs not showing GC growth. The GC mode had rates and activation energies similar to those of a polymorphic transformation observed near Tg. The GC mode was disrupted by the onset of the liquid's structural relaxation but could persist well above Tg (up to 1.15 Tg) in the form of fast-growing fibers. We consider various explanations for the GC mode and suggest that it is solid-state transformation enabled by local molecular motions native to the glassy state and disrupted by the liquid's structural relaxation (the alpha process).

Molecular liquids can develop a fast mode of crystal growth ("GC growth") near the glass transition temperature. This phenomenon remains imperfectly understood with several explanations proposed. We report that GC growth in o-terphenyl conserves the overall volume, despite a 5% higher density of the crystal, and produces fine crystal grains with the same unit cell as normally grown crystals. These results indicate that GC growth continuously creates voids and free surfaces, possibly by fracture. This aspect of the phenomenon has not been considered by previous treatments and is a difficulty for those models that hypothesize a 5% strain without voids. Given the existence of even faster crystal growth on the free surface of molecular glasses, we consider the possibility that GC growth is facilitated by fracture and surface mobility. This notion has support from the fact that GC growth and surface growth are both highly correlated with surface diffusivity and with fast crystal growth along preformed cracks in the glass.

A remarkable property of certain glass-forming liquids is that a fast mode of crystal growth is suddenly activated near the glass transition temperature, Tg, and continues in the glassy state. This mode of growth, termed GC (glass-crystal), is so fast that it is not limited by molecular diffusion in the bulk liquid. We have studied the GC growth by growing multiple crystal polymorphs from the liquid of ROY, currently the top system for the number of coexisting polymorphs of known structures. We observed a new feature of GC growth that conflicts with its current description in the literature. We found that the GC mode is not truly a new growth mode suddenly appearing near Tg but one already existing in the equilibrium liquid up to approximately 1.15 Tg, in the form of fast-growing fibers. This finding is relevant to testing different explanations for GC growth and favors the view that GC growth is enabled by molecular motions that are native to the glass but still persist in the viscous liquid.

o-terphenyl is one of the organic liquids in which a fast mode of crystal growth is activated near the glass transition temperature T(g) and continues deep in the glassy state. This growth mode, termed glass-crystal (GC), is not limited by molecular diffusion in the bulk liquid, in contrast to the diffusion-controlled growth at higher temperatures. The GC mode has been previously described as abruptly emerging near T(g) and having a constant growth rate at a fixed temperature, two features important for testing its various explanations. We report here that the GC mode already exists in the equilibrium liquid of o-terphenyl up to 1.15T(g) (T(g)=246 K) in the form of loose, fast-growing fibers and that its growth rate is constant at T(g)+2 K, but decreases by 30% in 10 h at T(g)-13 K, during which time the glass' fictive temperature decreases by 6 K. The slow down of GC growth becomes less noticeable over time so that fast growth is still observable after long annealing. The fiber growth, similar to the fully activated GC growth that yields compact spherulites, is also not limited by bulk diffusion. Crystal growth in the GC mode has a comparable activation energy as liquid desorption but a much faster rate, properties in common with polymorphic conversions. The time dependence of GC growth is not readily explained by the effect of physical aging on the thermodynamic driving force of crystallization, the liquid desorption, the primary structural relaxation, or a secondary relaxation. The secondary dielectric relaxation observed by dielectric spectroscopy in glassy o-terphenyl disappears too quickly for its molecular motions to be responsible for GC growth.