The Growth of Crystals at Low Supersaturation

Several \\~ell-sllbstantiated examples of both two-dimensional nucleation (ZDN) and multiple scre\v dislocation growth (MSDG) are described. The relations derived in Part I permit quantitative estimates of the edge energy parameter b in some cases, and confirm former interpretations of growth behaviour, except in the two instances now specified. The use of edge rather than surface energy as the controlling parameter for 2DN permits the whisker to platelet transition for Hg (as well as for Cd, Zn. Ag and CdS) to be ascribed to 2DN on edge planes of whiskers growing with XII axial screw dislocation. The linearly saturation-dependent growth rate of alum from solution below cx 1.01, formerly thought to be unimpeded SDG is shown to occur below a transitional region of impeded growth between x I.1 and 1.01. and to be itself impeded. This second linear law may be due tither to a discrete array of multiple dislocations on each face, or to continued growth of higher energy plants after low energy planes have stopped growing. Other examples of MSDG are salol from the melt, at an impeded rate almost linear with (aI). and CloHx, P,. IL and K from the vapour. The observed devclopmcnt of I I IZ} faces on non-growing K crystals is not explicable by crystal growth theory. Dislocation-free [OOOI} faces of ice crystals, and I I I I]. faces of Gc groM from the melt by 2DN. In both cases the nucleation barrier is lov,ered by entropy terms in the nucleus edge energy. The growth of NaCIOz and sucrose from solution may be due to 2DN or SDG.