Variable-temperature 1D and 2D Raman spectra, as well as powder X-ray diffraction patterns, reveal the appearance of two peaks that can attributed to a two-step phase transition, and the extent of the second-step phase transition increases with milling time (or defect density). Thermal analyses of polycrystalline powders show two endothermic peaks in the α-to- β phase transition (and two exothermic peaks for the reverse transition), in contrast with one thermal event observed for single crystals. Here, the effects of crystal defects on the solid-to-solid phase transformations between DL-methionine polymorphs α and β are investigated by means of experimental and computational approaches. However, such effects remain poorly understood. Ubiquitous defects in crystals may play an important role in the pathways of phase transitions. Understanding of solid-to-solid phase transition mechanisms in polymorphic systems is of critical importance for rigorous control over polymorph purity in the pharmaceutical industry to achieve the desired bioavailability and efficacy of drugs.
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