The formation of graphite/bromine I. Hysteresis of bromine insertion between the carbon hexagon layers

Abstract

When well-oriented graphite is exposed to bromine, at least three different regimes can be characterized. (i) On first taking up bromine from the vapour or from solutions in CCl$_4$, graphite appears to form crystal compounds of metastable structure with practically full expansion for only about one-third of the limiting uptake. (ii) Given sufficient time, all the solids attain a second, more stable, group of structures with molar volumes in linear relation to the bromine uptake. (iii) When the uptake from solutions of bromine in carbon tetrachloride is studied at various concentrations and at different temperatures, critical thresholds are found below which crystal compound formation is negligible. These thresholds can be correlated to some extent with the degree of crystal perfection. Above the threshold, the limiting uptake of bromine depends on the partial pressure of bromine vapour according to a simple logarithmic law. Factors leading to irreversibility on inserting or removing bromine between the layers are discussed. Various portions of the graphite appear to have their carbon hexagon layers pinned, or prevented in some way from separating so as to intercalate layers of bromine to form crystal compounds. Results are interpreted on the basis of effects that promote unpinning the layers.