Determining the orientations of BrF3 and FeBr3 molecules in graphite fluoride matrices using the XANES and EXAFS polarization dependences
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Synchrotron radiation was used to measure the EXAFS and XANES polarization dependences for intercalation compounds of graphite fluoride. An approach is developed which allows one to analyze the orientation of molecules of arbitrary shapes using XANES and EXAFS data. Analyzing the orientation dependences of BrK XANES spectra for the T-shaped BrF3 molecules, we determined possible combinations and admissible ranges of angles between the normal to the graphite fluoride matrix planes and the Br−F bond directions (α=52–90°, β=27–82°) and between the normal to the matrix planes and the molecular planes (γ=27–53°). The average orientation angles obtained by the combined analysis of the EXAFS and XANES data are as follows: α=62±1.5°, β=58±1.5°, γ=45±1.5°. The interatomic distances Br−F, Br−Br, and Fe−Br are determined. It is established that thermal treatment, which recovers the X-ray diffraction pattern from the unfilled matrix, does not affect the predominant orientation of the BrF3 molecules. This suggests that the thermally treated graphite fluoride matrix contains thin layers of ordered molecules. The absence of the polarization dependence of the spectra of FeBr3 in graphite fluoride allows the assumption that the molecular planes are oriented with respect to the normal to the matrix planes at a “magic” angle of 35°.
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