Interlaced spiral growth and step splitting on a steroid crystal

Abstract

This paper presents detailed ex situ observations of spiral patterns on the ( 0 1 ¯ 0 ) face of the stable polymorph of the 7 αMNa steroid, using atomic force microscopy. In contrast to the opposite (0 1 0) face, which is rough, the ( 0 1 ¯ 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 ¯ 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, 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 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.