Optical studies of the growth from solution of some long-chain organic crystals

Abstract

Growth patterns upon the (001) faces of crystals of stearic acid, CH3 (CH2)16 COOH, and behenic acid CH3 (CH2)20 COOH, are described. The crystals were grown from solution in chloroform at temporatures~268°K, and studied by optical, interferometric and electron microscopic techniques. The crystals were found to be imperfect and to have grown by a spiral mechanism, in accordance with the theory proposed by Burton, Cabrera and Frank. Two independent methods were used to determine the height of the spiral steps. Inter-ferometric measurements showed that the mean step-height was an integral multiple of c sin beta. There were four exceptions, in which the step-height was 1/2 N c sin beta where N was an odd integer. Electron microscopic measurements on crystals of behenic acid revealed mainly bimolecular growth steps (c sinbeta). Six examples of trimolecular steps 9delta/2 c sin beta) were also found. These results showed that Frank's hypothesis that the normal component of the Burgers vector of a dislocation is equal to an interplanar spacing of the lattice was not always valid. An approximate calculation showed that many of these crystals had grown at supersaturations as low as 2%. Evidence of unit slip and internal slip was given .It was shown that internal slip did not perfectly replicate the underlying surface structure; internal slip occurred through thicknesses as much as 1000 - 1500 A°. Patterns due to internal slip also suggested that the dislocation lines were not necessarily parallel to the c-axis or perpendicular to (001). The shape of the growth spirals was discussed. It was found to depend upon the crystal structure, the temperature at growth, and the inclination of the molecules to the basal plane. It was shown that layer disorders occur in crystals of the fatty acids. These disorders were repeated by spiral growth, leading to polytypism or polysynthetic twinning, and characteristic interlaced growth patterns. The component layers were related by rotations through an angle phi = 180°, 106°, or 74°. When phi = 106° or 74° the layers were in twinned orientation. Occasionally random values of phi occurred. This, together with observations of helicoidal crystals suggested that the dislocations were due to non-uniform distributions of stresses leading to buckle and shear of the thin crystal plates, as proposed by Frank.