Wellhyphen;known ideas suggest that the mass ratio of the constituents and the ionicity of a diatomic lattice will influence certain features of its vibrational spectrum: The splitting between the optical and acoustical branches increases with increasing mass ratio and the energy of the longitudinal optical branch is increased relative to that of the transverse optical branch by increasing ionicity. These effects are found to exist in the phonon frequencies which contribute to the lattice ``combination bands'' of zincblendehyphen;type semiconductors.
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