Temperature Dependence of the Long-Wavelength Optical Phonons in Diamond

We investigate the effects of temperature in the range 10-1000°K on the frequency and line-width of the $\overrightarrow{\mathrm{q}}\simeq 0$ optical phonons in diamond, using Raman-scattering techniques. Comparison is made between the present results and those obtained experimentally and theoretically by other workers. As the temperature increases, the broadening of the line is determined by an increasing number of channels through which the mode decays into pairs of acoustical modes. Beyond 850°K it is found that the crystal exhibits higher-order anharmonicity. It is also established that the frequency shift is largely due to thermal expansion, and that the scattering occurs under conditions of phonon-population equilibrium.