Lattice imperfections in organic solids. Part 2.—Sucrose

Abstract

Defects in single crystals of solution-grown surcrose have been studied using optical and electron microscopy. From an examination of the etch figures (produced by aqueous ethanol) and the cleavage patterns on {100} faces it is shown that dislocations may move on the {h10} planes in the 〈001〉 directions and on the {h01} planes in 〈100〉 directions. These dislocations may form loops and polygonized arrays and certain kinds of low-angle boundaries. There is a prism of enhanced imperfection within each crystal where the dislocation density is ca. 106 cm–2. Outside the prism, which is bounded by two planes of high Miller indices, the dislocation density is 103 cm–2. The Burgers vector of the screw dislocations emergent at {100} faces can range from a few A to 5000 A. The cores of these dislocations consist of a clolumn of water impurity which may be liberated on heating and by mechanical means. Further heating results in the formation of decomposition “volcanoes” on the surface—it is difficult to decide whether these coincide with dislocations—and, at a later stage, caramelization takes place. Caramelization tends to occur preferentially within the prism of enhanced imperfection, and its course of development in the 〈010〉 direction may be followed microscopically. Friedel's formula concerning the surface tension of cleavage steps is obeyed.