Commensurate-Incommensurate and Melting Transitions in Bromine-Intercalated Single Crystal Kish Graphite

Abstract

ABSTRACTWe summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br2 compounds. The experiments are aimed at understanding the intraplanar and interplanar correlations of the intercalate bromine as a function of temperature. Using in-situ high resolution x-ray scattering techniques, we have studied stage-4 graphite-Br2 under equilibrium conditions, where the intercalant is in the commensurate phase, incommensurate phase, and fluid phase. We demonstrate that the transitions between these phases are model examples of phase transitions in quasi two-dimensional systems. The coherently ordered in-plane bromine regions exceed 10000 Å in size in both commensurate and incommensurate phases. A commensurateincommensurate transition in the 7–fold direction is observed from a centered (√3 × 7) phase to a stripe domain phase. The domain wall density exhibits a power-law behavior in temperature with an exponent of 0.50 ± 0.02. We have also observed power-law lineshapes for bromine superlattice peaks in the incommensurate phase due to the rigorous absence of true long range order. The intercalate layer appears to exhibit an anisotropic melting transition from an incommensurate solid to a novel fluid phase.