The multiphonon absorption spectra of MgO and CaO single crystals have been recorded, and analysed in terms of the infrared active sum modes of lattice vibrations. A critical point analysis was carried out using the dispersion curves obtained from previous neutron scattering studies and the 'breathing shell' model, and it produced similar mode combination patterns for both substances. The two main peaks I and II (846.5 cm -1 and 981 cm -1 for MgO; 628 cm -1 and 738 cm -1 for CaO) in the two- phonon region are interpreted as the sum modes W' 2 +W" 3 at W and Sigma 1 LO+ Sigma 3 TO near X respectively, in both crystals. Three- phonon bands were observed for both MgO and CaO and a four- phonon band in MgO, all at wavenumbers in agreement with theoretical predictions.
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Gourley et al. (1973) studied this question.
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