The growth and perfection of crystals of ammonium dihydrogen orthophosphate, NH4H2PO4. I. Dislocation characterization in self-nucleated crystals

Abstract

A combined theoretical analysis and experimental study has been made of the nature of dislocations which propagate in the prismatic and pyramidal growth sectors of ammonium dihydrogen phosphate (ADP). In general the line energies of the dislocations formed in both the sectors are less than 45 eV nm−1. A few higher-energy dislocations (123 eV nm−1) were characterized in the prismatic sectors. The line directions of dislocations which are formed in the pyramidal sectors are such that they propagate towards the growth-sector boundaries and cross into the prismatic sectors. Alternatively, they follow the growth-sector boundaries and emerge at the intersection of adjacent faces. Few, if any, dislocations propagate within these sectors. Consequently, although dislocation-controlled mechanisms of growth may contribute to the growth of the {101} faces of ADP in the early stages of growth, other mechanisms will dominate at longer times. In contrast, the prismatic sectors are more imperfect, containing both screw and mixed dislocations. For these sectors spiral-growth mechanisms are more likely to dominate at all stages of growth.