Tracer studies on the solid state

Abstract

A study of self-diffusion in anthracene single crystals has been carried out using a radioactive tracer technique. A method was developed for the growth of large single crystals of anthracene from the melt, starting from the Bridgman technique. On to one surface of the crystals was evaporated a deposit of anthracene - 9 - carbon-14, prepared by a seven stage synthesis from barium 14-carbonate, and the crystals and their deposits were heated at constant temperatures in the range 150 -190 G for periods up to 230 hours. The crystals were then sectioned parallel to the initial active face, using a calibrated, hand operated, lathe. The quantity of radioactive material which had diffused into each section was determined by quantitative combustion of the sections to carbon dioxide in an "Empty Tube" rapid combustion apparatus, followed by the determination of the disintegration rate of this gas in a gas counting tube, using carbon disulphide as charge transfer agent. Penetration was found up to a depth of 4 x 10(^-2) cms. It was found that two concurrent diffusion processes were occurring in the crystal. One of these involved only a small proportion of the total diffusing activity and may represent diffusion along intersecting dislocations in the crystal. The second process involved most of the diffusing activity and was thought to represent bulk diffusion in the crystal The temperature dependance of the diffusion coefficient for the second process is given lay the equation. D = (1.31 ± 1.13) x l0(^ll)e (^–(42,400 ± 1,200)/RT) cm. (^2) per second. The pre-exponential factor and activation energy for this process are unusually high. This leads to the conclusion that a co-operative diffusion phenomenon is occurring in the crystal, in which diffusion occurs by way of vacant lattice sites, and that the loosening or premelting of from four to six molecules occurs in the neighbourhood of the vacancy during diffusion. This is one of the few studies of self-diffusion in molecular crystals.