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Infrared absorption bands characteristic of the crystal lattice were investigated for germanium and silicon. The absorption was found to be insensitive to lattice imperfections due to impurities (10^18 cm^-3) and due to disorders (10^19 cm^-3) produced by nucleon irradiation. It did vary with temperature in the same way as the calculated mean-square displacement of the atoms. Variations of absorption with temperature were also studied for two specimens of type I diamond. The bands characteristic of type II diamonds showed similar temperature dependence, whereas the long-wavelength bands were temperature insensitive. It appears that the observed absorption in germanium and silicon corresponds to the absorption characteristic of type II diamond and is made possible by the thermal vibration of the atoms. The long-wavelength bands in type I diamonds may be due to impurities.
Collins et al. (1954) studied this question.