The study of crystal growth with the electron microscope IV. The nucleation and growth of n -propyl n -pentacontanoate

Abstract

The study of long-chain paraffin derivatives in the electron microscope has been extended to include the fatty acid ester n -propyl n -pentacontanoate, n -C 53 H 106 o 2 . Examination of crystals prepared by previously described methods has given information on two further aspects of crystal growth. In the first place micrographs have been obtained at a much earlier stage in the growth of the crystals, before recognizable growth-steps have developed. It would appear that growth arises from sheet nuclei consisting of small condensed monomolecular films of 0.25 to 20μ 2 area. Screw dislocations arise at the edge of these sheets through the condensation of molecules in positions of incomplete lattice register. Growth thereafter continues around these dislocation centres, and the resultant spiral is built in towards the centre of the crystal face. This information modifies the Frank (1951) concept of a buckling mechanism for the formation of screw dislocations in crystals, since it places the emphasis on the importance of edge condensation in the formation of lattice disorder and minimizes the importance of strain in the growing perfect lattice. The second finding has been that these crystals frequently occur with a twist boundary parallel to {001}. This effect cannot properly be described as rotational slip of the type discussed by Wilman (1951), since it does not arise through deformation of an already formed crystal but occurs during growth itself. This twist boundary is penetrated by a screw dislocation or by a group of screw dislocations.