Role of dislocations in the growth of single crystals of potash alum

Abstract

Using a combination of growth kinetics and X-ray topographic studies an assessment has been made of the role of growth dislocations in the growth process on the {111} and {110} surfaces of seeded potash alum single crystals.

{111} Growth sectors contain few dislocations which initiate at the seed interface. Most growth dislocations enter these sections at later stages of growth from adjacent {110} and {100} sections. These dislocations appear to have little influence on the growth process, the kinetics and mechanism being defined at an early stage of growth to give a constant growth rate.

The {110} sectors contain dislocations of all types which initiate at the seed crystal interface and influence the growth process. Identification of the numbers of different types of growth dislocation and the correlation of their density with growth rate shows no distinct and regular variation of growth rate with the number of screw and/or mixed dislocations. Edge dislocations must be included in the total to yield a regular and potentially dominant influence. Studies of the supersaturation dependence of the growth rate of {110} faces yields a behaviour consistent with growth by a two-dimensional nucleation mechanism.

This is additional evidence to that already noted in growth of the {100} sectors for a strong contributory role of edge dislocations to crystal growth in some sectors of this material.