ROUGH FLAT ROUGH TRANSITION OF CRYSTAL-SURFACES

ABOVE a characteristic temperature, the free energy required to form a step on the surface of a growing crystal may fall to zero; the surface then undergoes a transition from smooth (faceted) to irregular (rough) growth. Here we describe studies of the growth of n-C21H44 crystals from n-hexane solutions close to the roughening transition. Beginning with equilibrium crystal growth at a temperature slightly above the roughening transition, we introduce progressively greater supersaturations by lowering the temperature. Observations of the growth morphology reveal a transition from rough to smooth growth, followed at greater supersaturations by a transition back to rough growth. To our knowledge, such a sequence of rough-smooth-rough crystal growth has not been reported before. Our measurements of the step free energy of the {110} faces indicate that it vanishes discontinuously at the equilibrium roughening transition temperature, in contrast to the continuous (critical) behaviour predicted by theoretical models 1-11.