Evidence for spiral growth on the pyramidal faces of KDP and ADP single crystals

Abstract

Microscopic observations of the pyramidal {101} faces of KDP (KH 2PO 4) and ADP (NH 4H 2PO 4) single crystals grown from aqueous solutions revealed that three different types of growth features are possible: growth hillocks, cooperating growth spirals with macro step heights of a few hundredsÅngströms and nucleation. From the correspondence between the locations of the growth hillocks and the etch pits formed after slightly dissolving the crystal in water, the growth hillocks could be identified as growth spirals, mostly originating from single dislocations. The growth spirals consisting of macro steps could be related to dislocation bundles. These bundles are often formed due to lattice closure errors after the formation of a liquid inclusion. Some typical phenomena as pseudo eccentricity of the growth spirals, step retardation due to strain fields and rhythmical bunching have been observed. In the case of highly perfect KDP crystals with hardly no dislocations ending on the {101} faces, crystal growth is governed by a nucleation mechanism.