The Phonon Distribution of Graphite at High Temperatures

The phonon spectrum of graphite is notable for the high energy of some of its modes; the cut-off in the frequency distribution is about 0.22 eV (2500°K) and it is difficult to measure accurately the whole of the distribution by neutron scattering experiments on samples at room temperature. Raising the temperature of the sample, however, increases the probability of high energy gains by the neutron. Therefore the inelastic scattering of slow neutrons by graphite at 1800°K has been measured using a phased rotor spectrometer. Energy transfers covering the whole range of the frequency distribution are observed with clearly defined peaks. The results are compared with theoretical predictions and show good agreement. The incoherent approximation is shown to be reasonably valid at this high temperature and a smoothed frequency distribution is given which allows the calculation of scattering cross-sections in agreement with data measured at other temperatures. (author)