Martensitic phase transitions in molecular crystals – p-dichlorobenzene

β-Phase p-dichlorobenzene bicrystals, mechanically deformed βsingle crystals, and crystals undergoing a β → α phase transformation have been examined optically and by X-ray diffraction. Frequently occurring boundaries between two crystals and the mutual orientations of the crystals may be explained by an epitaxic relation between two simple faces of each of the crystals. It is proposed that the mechanism of nucleation of the daughter (α) phase involves a balance between the coherence range of a series of fluctuations in molecular orientation and a critical nucleus size (determined by a further balance of bulk and surface forces in the nucleus). This is sufficient to explain α-nuclei orientations, association of nucleation with defects, and sensitivity of nucleation to temperature, doping, and deuteration. In some crystals the growth of daughter nuclei (both α and β) is rapid. Some crystal defects can slow growth and increase nucleation rates drastically. This work suggests that the previous distinction between martensitic and other nucleation- growth mechanisms in molecular crystals is an oversimplification. Molecular first-order phase transitions are fundamentally martensitic. Obscuring factors such as the large number of alternative pathways, and the extension in range of possible nuclei orientations caused by defects are briefly discussed.