Electron microscopic studies of extended defects in organic molecular crystals. Part 1.—p-Terphenyl

Abstract

Transmission electron microscopy has been utilized to reveal directly the nature of the dislocations that occur in solution- and vapour-grown thin crystals of p-terphenyl. The identity of the linear defects was established chiefly by diffraction contrast using bright-field images, but it also proved possible to employ, to a limited degree, real-space crystallography. Many of the slip systems deduced to be present in simple aromatic solids by dislocation-etching and topographical studies have been more fully characterized in this study {e.g. (001)[010], (001)[110], (100)[010] and (100)[001]}. Of special interest is the identification of the hitherto undetected slip system (001)[12w](with w probably zero). At the cores of such dislocations, in aromatic solids crystallizing in the P2₁/a space-group, molecules within the structure adopt a space-symmetry which indicates that incipient dimers occur recurrently. This is relevant in the solid-state chemical behaviour of anthracene since such dislocations can rationally account both for the production of topochemically forbidden dianthracene upon u.v. irradiation of the cyrstalline monomer and te trapping of triplet excitons in single-crystals of anthracene.