The symmetry of phonon spectra in crystals with sublattices

A method is presented for investigation into the symmetry of phonon spectra of complex compounds composed of the Bravais sublattices. It is shown that quasi-degeneracy in the phonon spectra of crystals containing sublattices with the symmetry higher than that of the crystallographic group of the compound results from this high symmetry. The mechanism responsible for the translational quasi-degeneracy has peculiar features for various structures. For substitution structures in a highly symmetrical lattice, the translational quasi-degeneracy is caused by convolution of the total phonon spectrum of the highly symmetrical structure. The convolution of the phonon spectra in crystals containing sublattices of different Bravais types occurs separately for each sublattice. Quasi-degeneracy also occurs in the compounds containing sublattices whose point symmetry is higher than that of the crystal structure. The method is illustrated by the example of a one-dimensional structure containing sublattices.