NEW PERIODIC MORPHOLOGIES OBSERVED DURING DENDRITIC GROWTH OF AMMONIUM-CHLORIDE CRYSTALS IN THIN-LAYERS

We report novel morphologies observed during the growth of dendritic crystals from supersaturated solutions of NH4Cl and NH4Cl(CuSO4). These develop when the anisotropic crystal grows in a thin layer. The crystal shapes are distinctly periodic and exhibit long-range order; their growth front appears to be marginally stable. Crystals of these forms grow as three-dimensional structures in quasi-two-dimensional cells, and their dominant periodicity is determined quantitatively by the smallest dimension of the cell. We argue that the observed phenomena result from an interplay between the geometrical constraint of the cell and the kinetics of the phase transition at the interface. The observations are consistent with a model in which dendritic growth occurs at temperatures below the equilibrium roughening temperature of the dominant crystal facet, although kinetic roughening of the facet occurs.