EXAFS study of Br2-graphite compounds

S. M. Heald, E. A. Stern

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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.

Original languageEnglish
Pages (from-to)87
Number of pages1
JournalSynthetic Metals
Issue number1-2
StatePublished - 1980
Externally publishedYes


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