Polarized-x-ray-absorption studies of graphite intercalated-bromine compounds

Abstract

Details of both results and data analysis are given in the case of our polarized-x-ray-absorption experiments, using synchrotron radiation, on highly oriented pyrolytic graphite (HOPG)--based and graphite-fiber-based residual-bromine intercalation compounds. The effective angle which nearest-neighbor Br pairs make with crystallite graphite planes in some of these compounds, which was stated to be \ensuremath{\sim}20\ifmmode^\circ\else\textdegree\fi{} in an earlier article, is shown to be 16?(de\ifmmode\pm\else\textpm\fi{}4?(de---both Br-Br extended x-ray-absorption fine structure (EXAFS) and white-line features of the data are the basis of this result. We have also found that, whereas spherical averages of the areas under white-line spectra are independent of the choice of the material among all samples studied (including ${\mathrm{Br}}_{2}$ vapor), differences in similarly spherically averaged Br-Br EXAFS amplitudes are evident, especially between ${\mathrm{Br}}_{2}$ vapor and Br-graphite samples. We show that the latter differences which correspond to a coordination number less than one in Br-graphite are not due to either Gaussian or non-Gaussian (up to ${k}^{4}$ terms) Debye-Waller effects. In addition, we discuss the extraction of Br-C EXAFS and present results of model calculations of Br-C EXAFS, where several different structural models for the Br sites are considered. We also discuss thermal effects and their relation to known Br sublattice phase-transition behavior, based on our measurements at room temperature, 360 K, and 400 K.