Anomalous thermal expansion of the interlayer spacing of graphite-bromine residue compound at high temperatures

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The thermal expansion behavior of the interlayer spacings of artificial graphite and graphite-bromine residue compounds was investigated by using a high temperature X-ray diffractometer at temperatures between room temperature and 800°C.<BR>The c-spacing of the artificial graphite sample used was confirmed to have an almost linear thermal expansion up to 800°C, and the calculated linear expansion coefficient agreed well with the values in literatures. On the other hand, an anomalous hysteresis was observed on the graphite-bromine residue compound (1.2 atm. % bromine). At room temperature, the c-spacing of the residue compound was a little larger than that of the graphite. When the temperature was increased, the normal expansion of the c-spacing of the residue compound showed a break at about 330°C, and apparently negative thermal expansion (contraction) was observed up to 500°C. Over this temperature, the c-spacing of the residue compound became nearly equal to that of the graphite. On cooling, the anomaly on reverse direction occured at temperatures between 330°C and 200°C.<BR>The observed behavior is explained as follows: the bromine molecules, which would exist in the interstices of graphite layers at lower temperatures, would become unstable above 330°C and would migrate from the interstices to crystal imperfections. This trans formation would proceed reversibly on cooling with some hysteresis.
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