Anomalous properties and structure of graphite-bromine residue compound at high temperatures

The thermal expansion of the interlayer spacing and the intensity ratio $\text{I(004)}\text{I(002)}$ of the diffraction lines of graphite-bromine residue compounds were measured at temperatures between room temperature and 700°C by using a high temperature X-ray diffractometer. At about 330°C, both the c-spacing and the intensity ratio of diffraction lines were found to show a break. That is, the r-spacing of the residue compound which shows normal linear expansion below 330°C, has a considerably smaller thermal expansion coefficient between 330 and 500°C. Over this temperature, the c-spacing of the residue compound becomes nearly equal to that of original graphite. On cooling, the anomaly in the reverse direction occurs between 330 and 200°C. The intensity ratio of the diffraction lines of the residue compound is smaller below 330°C as compared with that of original graphite, but above 500°C, both are almost the same. The thermal diffusivity and the heat-capacity studies were also performed in the same temperature range. The thermal diffusivity curve shows a break, and the heat-capacity curve has a small hump around 330°C. At temperatures higher than 330°C, both of these properties agree with those of original graphite. The structure of graphite-bromine residue compound and the nature of its change at high temperatures are discussed, and a model to explain all of these peculiar behavior of the residue compound is proposed.