Effet hall et diamagnetisme de composes residuels pyrocarbone-bore-halogene

Abstract

In order to clarify the electronic structure of halogen residue compounds, particularly the amount of charge transfer that occurs between the carbon π band and the intercalated species, bromine and iodine chloride were intercalated into pyrolytic carbons with various levels of boron doping, and the Hall effect and diamagnetic anisotropy of the resulting residue compounds were studied. The positive Hall coefficient is always independent of temperature and yields the concentration of holes in the π band, while the amount of intercalated halogen can be obtained from weight variation measurements. Table 1 shows the Hall coefficients and hole concentrations of the boronated pyrocarbons before intercalation, and Table 2 gives the results of measurements performed on residue compounds between 77 and 300 K. Increasing heat treatments reduce the amounts of halogen (Table 3). From these various data it can be stated that the halogen content of the residue compounds does not depend on the boron doping and the resulting initial position of the Fermi level (see Tables 2 and 3). The charge transfer from carbon to halogen is always found to be quite small: 0.02–0.07 electronic charge per Br 2 or ICl molecule (Fig. 1). The charge transfer also varies if the compounds undergo thermal cycles without loss of halogen. This was shown by in situ measurements of the Hall coefficient and diamagnetic anisotropy during such cycles (Figs. 2–6). Successive intercalations and deintercalations, as suggested by the Marchand-Rouillon model[5,6], are responsible for the hysteresis cycles observed[1–7] with many properties of these compounds.