AFM observation of barium nitrate {111} and {100} faces: spiral growth and two-dimensional nucleation growth

The growth mechanisms of the {1 1 1} and {1 0 0} faces of Ba(NO3)(2) crystals growing from aqueous solutions were investigated by ex situ atomic force microscopy. Growth hillocks induced by dislocations and growth islands formed via 2D nucleation were observed on both faces. The thinnest steps observed on the {1 1 1} and {1 0 0} faces were, irrespective of step sources, d(1 1 1) = 4.7 Angstrom and d(2 0 0) = 41 Angstrom in height, respectively. These correspond to the elementary growth layers expected in Bravais-Friedel-Donnay-Harker (BFDH) and Hartman-Perdok (i.e. periodic bond chain) theories. The spiral hillocks on the {1 0 0} face consist of double elementary layers. On {1 1 1} faces, three kinds of spiral layers arising from single dislocations were discerned: single, double and triple elementary layers, which can be produced by dislocations with Burgers vectors b = [1 0 0], [1 1 0] and [1 1 1], respectively. It was observed that the multiple spiral layers tend to split into elementary steps at the spiral centres, which can be explained by entropic repulsion. It was also found that several spiral centres are accompanied with hollow cores and that the diameters of these cores vary with the number of spiral arms connected with the central dislocation. The numerous 2D nuclei of elementary height found between the spiral arms were probably created in the short period of very high supersaturation during separation of the crystal from the solution. (C) 1998 Elsevier Science B.V. All rights reserved.