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COMMENSURATE-INCOMMENSURATE AND MELTING TRANSITIONS IN BROMINE-INTERCALATED SINGLE CRYSTAL KISH GRAPHITE A. ERBIL, A.R. KORTAN, R.J. BIRGENEAU and M.S. DRESSELHAUS? Department of Physics and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

ABSTRACT We summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br 2 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-Br 2 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 A in size in both commensurate and incommensurate phases. A commensurateincommensurate transition in the 7-fold direction is observed from a centered (V3 x 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.

INTRODUCTION Graphite-Br 2 intercalation compounds are of considerable scientific interest because of the existence of several structural orderings in the intercalate bromine layers and the occurrence of phase transitions between these ordered phases. The types of in-plane orderings prevalent in graphite-Br 2 compounds are commensurate, incommensurate and fluid phases. Several independent experiments on graphite-Br 2 compounds exhibit anomalous temperature-dependent behavior above room temperature. X-ray diffraction [11 from the a-face of a desorbed graphiteBr 2 compound has provided evidence for the existence of a commensurateincommensurate transition (C-IT) at -340 K, in addition to a melting transition at 373.7 K in the intercalate layer. The temperature dependence of the Raman lineshape, linewidth, frequency and intensity of the intercalate bromine molecular stretch mode [21 of a graphite-Br 2 compound has yielded information on C-IT and melting transitions, consistent with these x-ray experiments. In this paper we summarize the results of an x-ray scattering study of the C-IT and melting transitions in stage-4 bromine intercalated graphite, C2 8 Br 2 . The stage-4 bromine-intercalated compound was chosen for the study because of its weak interplanar interaction and resulting quasi two-dimensional (2-D)

behavior. talso Department of Electrical Engineering and Computer Science.

Hat. Res.

Soc.

Symp. Proc. Vol.

20 (1983)

@Elsevier Science Publishing Co.,

Inc.

22