The theory of mixed photon-surface phonon excitations outlined by Hopfield (see abstr. A3419 of 1959), and applied to finite ionic crystals by Englman and Ruppin (see abstr. A44287 of 1968), is used to interpret some forbidden 'extra modes' of the corundum phonon spectrum.
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Rosario Sergio Cataliotti (1974) studied this question.
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