The anharmonicity of infrared vibration modes has been studied in beryl crystals by measurements of the temperature dependence of reflectivity. The infrared reflectivity data from 280 to 1500 cm−1 in the temperature range 300 to 1300 °K, were analyzed with the classical oscillator model. The comparison between the results of quantum theories and the dependence of observed damping on temperature shows that the cubic and quartic terms contribute to the anharmonicity of a set of modes, whereas another set of modes exhibits only a cubic anharmonic behaviour. In the latter case, a part of the modes are known as internal.
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Gervais et al. (1972) studied this question.
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