C. Sweegers, M. Plomp, H. C. de Coninck, H. Meekes, W. J. P. van Enckevort, I. D. K. Hiralalb and A. Rijkeboer
Abstract
Optical, scanning electron and atomic force microscopy were used to
examine the surface topography of gibbsite ((gamma)-Al(OH)3)
crystals grown from supersaturated caustic soda solutions. Several growth
phenomena like monomolecular and higher steps, growth hillocks, contact
nucleation of steps and planar faults were observed. It turned out that
the growth features observed are related to the defect structures of the
different gibbsite crystal morphologies found. Single crystalline lozenges
of a few um thickness have hillocks on the basal {0 0 1} faces, which are
related to one or a few dislocations. For ultrathin lozenges (few tens
of nm thick and an aspect ratio larger than 1000), the surface morphology
is flawless and no dislocation sources emerging the {0 0 1} surface were
found. Here growth proceeds by 2D nucleation. The second type of crystals,
sixfold twinned hexagons, has irregular {0 0 1} surfaces as a result of
many defects. The introduction of defects leads to enhanced growth parallel
to the c-axis. Lateral expansion proceeds by a 2D nucleation of the fast
growing {1 0 0} side faces and the enhanced 2D nucleation at the re-entrant
corners at the outcrops of twin planes. Crystals of the third major crystal
morphology found, i.e. prisms, also exhibit many defects. Mosaicity was
observed and related to the presence of misaligned crystallites or impurities.
The fact that the prismatic crystals do not show a pronounced lateral growth
as the hexagons do, suggests that besides defects also other factors influence
gibbsite crystal growth.