Surface topography of gibbsite crystals grown from aqueous sodium aluminate solutions

Abstract

Optical, scanning electron and atomic force microscopy were used to examine the surface topography of gibbsite (γ-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 μm 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.