Interlaced
spiral growth and step splitting on a steroid crystal
Stoica C (Stoica, C.), van Enckevort WJP (van Enckevort, W. J. P.),
Meekes H (Meekes, H.), Vlieg E (Vlieg, E.)
J. Cryst. Growth 299 (2007) 322-329
Abstract:
This paper presents detailed ex situ observations of spiral patterns on
the (0 (1) over bar 0) face of the stable polymorph of the 7 alpha MNa
steroid, using atomic force microscopy. In contrast to the opposite (0
10) face, which is rough, the (0 (1) over bar 0) face grows layer-wise
involving steps down to monomolecular height. The growth spirals show
symmetry induced interlacing, i.e. a regular splitting of d(010) steps
into half height d(020) steps as a consequence of the 2(1) screw axis
perpendicular to (0 (1) over bar0). In many cases differences in
advancement rate of symmetrically non-equivalent layers and entropic
repulsion leads to further step splitting generating steps of quarter
unit cell height, d(040). The observed step orientations are identical
to the strongest periodic bond chain directions within the thinnest
growth layer thickness. The screw dislocations b = [0 10] at the spiral
centers are marked by hollow cores. The edge dislocations ending on the
same face do not reveal hollow cores. This is a consequence of their
low stress fields, which is some 40 times smaller than that of the
screw dislocations. (c) 2007 Elsevier B.V. All rights reserved.