Abstract
A dispersion relation for long‐wave phonons and phonon‐polaritons in monoclinic polyatomic crystals is derived. It can be brought in the form of a generalized Fresnel equation for the wave normal referring to the principal dielectric axes at a fixed frequency in the visible. Angular dispersion of coupled phonon‐ and polariton‐modes in the crystallographic plane with directional dispersion of the dielectric axes, and the possibility of experiments are discussed.

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