Preroughening in organic crystals

The (011) surface of the naphthalene crystal structure is described by a simple restricted solid-on-solid model that Is applicable to a wide range of organic crystals. Despite the absence of next-nearest-neighbor bonds. the phase diagram of the model exhibits, besides flat and rough phases, a disordered flat phase. Monte Carlo simulations confirm this phase diagram. The presence of a disordered hat phase explains the sudden disappearance of the (011) facet in experimental growth shapes of naphthalene. The type of step-free energy associated with surface tilt decreases rapidly, at preroughening transitions. This leads to a rapid decrease in the nucleation barrier, which results in a sudden enhanced growth rate and, under certain conditions, to the complete disappearance of this facet in growth shapes.