EXAFS study of Br₂-graphite compounds

Extended X-Ray Absorption Fine Structure (EXAFS) measurements have been made on a variety of Br₂-graphite compounds including a low concentration residue compound and intercalation compounds ranging from 0.27 mol% Br₂ to approximately stage 3. The EXAFS spectra have distinct contributions from both BrBr and BrC distances. This work is concerned primarily with the fate of the Br₂ upon intercalation as revealed by changes in the BrBr EXAFS, although a limited analysis is made of the BrC contribution. A unique result of this work is the dicovery that intercalated Br₂ takes two forms distinguished by differing BrBr distances, and charge transfers. At high concentrations most of the molecules have a BrBr distance of 2.34±0.02 Å, which is slightly larger than the values found for adsorbed (2.31 Å) or free (2.28 Å) molecules. These molecules lie flat between the graphite layers. The second type have a BrBr distance of 2.53±0.03 Å and are randomly oriented with respect to the graphite layers. These expanded molecules predominate in the 0.27 mol% sample, but their number remains fixed as more Br₂ is added, and in the high concentration samples they are only a small fraction of the total Br₂. From these results it is reasonable to speculate that the expanded Br₂ is associated with the edges or defects in the graphite, possibly acting as a “can-opener” in preparing the layers for further intercalation. From a comparison of our results with work function measurements for adsorbed Br₂, the charge transferred to the molecules is estimated as 0.16 electrons for the intercalated form and ∼0.5 electrons for the expanded molecules. The existence of these two types of molecules can explain the apparent change of charge transfer with concentration found by some authors at low Br₂ concentrations.