Interlaced spiral growth and step splitting on a steroid crystal

This paper presents detailed ex situ observations of spiral patterns on the (01¯0) face of the stable polymorph of the 7αMNa steroid, using atomic force microscopy. In contrast to the opposite (0 1 0) face, which is rough, the (01¯0) face grows layer-wise involving steps down to monomolecular height. The growth spirals show symmetry induced interlacing, i.e. a regular splitting of d010 steps into half height d020 steps as a consequence of the 21 screw axis perpendicular to (01¯0). 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, d040. The observed step orientations are identical to the strongest periodic bond chain directions within the thinnest growth layer thickness. The screw dislocations b=[0 1 0] 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.