Rough Growth Behavior of a Polar Steroid Crystal: A Case of Polymorphic Self-Poisoning?
C. Stoica, P. Tinnemans, H. Meekes,* W. J. P. van Enckevort, and E. Vlieg

Abstract:
The polar shape of the stable polymorph of the steroid crystal 7MNa is a consequence of different growth rates in the opposite crystallographic directions [010] and [00]. Using various microscopic techniques, it is shown that the surface morphologies of the opposite (010) and (00) faces are completely different, regardless of the conditions of growth or etching. The (00) face is flat and grows via a spiral growth mechanism, whereas the (010) face and faces close to it are rough. In situ microscopy shows that in many cases these latter faces are hampered in growth. The aberrant growth behavior of this group of faces is explained to be a result of the presence of an ultrathin layer of the metastable polymorphic form covering the surface, even under conditions at which the bulk metastable form is undersaturated. This layer retards growth and introduces roughening.