Kinetics of Crystallization of Anthracene from the Vapor

Anthracene was purified by zone melting, chromatography and sublimation. It was again zone melted in a closed system within which it was transferred to a crystal-growth tube. A sublimation apparatus was constructed allowing direct, automatic control of temperature-difference δT between sublimand and crystallization site as well as photography of the growing crystal. In evacuated tubes and in the presence of propane at 1 torr, it was not possible to control nucleation at 120°C. In the presence of N2 at 1 torr, single nuclei were formed at δ T=9.5° ± 0.5° and these were grown to 1 cm diameter crystals of good optical quality. The basal plane of the crystals was the ab plane. Cinematographic observation of growth and evaporation at under-saturations and supersaturations up to 6% showed that growth rate R varies linearly with supersaturation σ. According to the Frank theory, R should be proportional to σ2 for small σ when growth takes place by a single dislocation. Evidently this very unlikely condition is not fulfilled.