Possible origin of enhanced crystal growth in a glass

Contrary to the common belief that a glassy state is stable against crystallization, Oguni and co-workers discovered unusual enhancement of the crystal growth rate of a few molecular liquids below the glass transition temperature Tg. We studied this phenomenon using o-terphenyl (OTP) and phenyl salicylate (salol), focusing on the roles of volume contraction ΔV upon crystallization. We confirmed enhancement of crystal growth below Tg for OTP. For salol, which has two kinds of crystal, the crystal growth rate below Tg is faster for a crystal of larger ΔV than for another crystal of smaller ΔV. Our results suggest the following physical scenario for the phenomenon: for a material having large ΔV, the volume contraction upon crystallization provides a crystal-glass interface with large excess free volume, which results in the mobility increase at the growth front and leads to enhancement of the crystal growth. This mechanism may be effective only below Tg, where density fluctuations cannot be quickly relaxed by hydrodynamic transport.