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Abstract
The present paper discusses special features in the formation of local vibrations in a crystal having an isotopic defect when these vibrations are in the regions of two and three-particle states of optical phonons and anharmonicity is taken into account. The existence of anharmonicity leads, in some cases, to a cardinal change in the spectrum of local states. Also considered are the conditions for the formation of local biphonons, as well as the available experimental data. Dispersion equations are derived in a number of special cases for triply-bound phonon complexes (triphonons), and conditions are discussed for their localization on an isotopic defect.