Two-Step Melting in a Bulk Crystal via Intermediate Metastable Liquid

The mechanism of melting is significant, as it links the structure and dynamics between crystal and liquid. In two dimensions, the crystal could first melt into a hexatic liquid before finally reaching a disordered liquid. However, such a hexatic liquid phase is unstable in three dimensions, and melting is recognized as a one-step process. Here we report a two-step melting process in a three-dimensional system, (S)-(+)-ibuprofen. The crystal melts through an indirect pathway that first transforms into an intermediate liquid phase exhibiting an extremely long lifetime followed by the transition to the ordinary liquid phase at a spinodal point with the occurrence of long-range fluctuations. Such observations suggest that the complexity of liquid could affect the transition pathway of melting. These results could lead us to hypothesize the existence of continuous melting in three dimensions.