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.