ROLE OF DISLOCATIONS IN THE GROWTH OF SINGLE-CRYSTALS OF POTASH ALUM

Using a combination of growth kinetics and X-ray topographic studies a n assessment has been made of the role of growth dislocations in the g rowth process on the {111} and {110} surfaces of seeded potash alum si ngle crystals. {111} Growth sectors contain few dislocations which ini tiate at the seed interface. Most growth dislocations enter these sect ions at later stages of growth from adjacent {110} and {100} sections. These dislocations appear to have little influence on the growth proc ess, the kinetics and mechanism being defined at an early stage of gro wth to give a constant growth rate. The {110} sectors contain dislocat ions of all types which initiate at the seed crystal interface and inf luence 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 wit h the number of screw and/or mixed dislocations. Edge dislocations mus t be included in the total to yield a regular and potentially dominant influence. Studies of the supersaturation dependence of the growth ra te of {110} faces yields a behaviour consistent with growth by a two-d imensional nucleation mechanism. This is additional evidence to that a lready noted in growth of the {100} sectors for a strong contributory role of edge dislocations to crystal growth in some sectors of this ma terial.