Synthetic Metals, 2 (1980) 109 - 120 109 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands PHASE TRANSITIONS IN GRAPHITE-HALOGENS* K. K. BARDHAN Department of Physics J. C. WU and D. D. L. CHUNG** Department of Electrical Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213 (U.S.A.) (Received June 7, 1980) Summary X-ray diffraction and differential scanning calorimetry (DSC) were used to study the phase transitions in graphite-halogens (Br2, IC1). In graphite- Br2, a reversible transition associated with the melting of the ordered inter- calate layers into liquidlike layers was observed at 373.7 K, and a transition corresponding to the change of streaks to spots in the (0001) zone electron diffraction pattern on cooling was observed at 226 K. Moreover, a transition at 277 K has been tentatively interpreted as being due to a phase transition associated with the stacking order of the intercalate layers. In addition, in graphite-Br2, the first observation was made of a phase transition involving the shifting of various X-ray diffraction peaks in various directions at ~ 340 K. This transition is tentatively interpreted as being due to a commensurate- incommensurate phase transition. In graphite-IC1, reversible transitions were observed at 307 and 314 K. The 314 K peak is associated with the melting of the ordered intercalate layers into liquidlike layers; the 307 K peak is due to a phase transition associated with the stacking order of the intercalate layers. Introduction Graphite intercalation compounds exhibit phase transitions which affect the stability [1] and the properties [2, 3] of these compounds. Therefore, information on the phase transitions is essential for the application of graph- ite intercalation compounds at various temperatures. This paper focuses on the use of X-ray diffraction and differential scanning calorimetry (DSC) to study the phase transitions in graphite- *Research sponsored by the Ceramics Program of the National Science Foundation, Grant No. DMR-7926242. **Also in the Department of Metallurgy and Materials Science.