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.