The simplified model of the optical phonon dispersion has been analyzed in the case of binary semiconducting compound. As it is well known, the typical solution describing vibrations of one-dimensional atomic chain composed of two types of ions exhibits the maximal value of the optical phonon frequency corresponding to the wavenumber q = 0. This behavior of optical phonon modes is an excellent model of lattice vibrations for many important semiconductors, such as GaAs or ZnTe.
Our paper demonstrates that apart from a typical solution there exists also another one. Under particular conditions for selected parameters describing the constituent ions, the optical phonon dispersion may be completely flat or even can change the curvature (point q = 0 corresponds in this case to the frequency minimum). The “anomalous” behavior of the LO phonon mode have been observed by inelastic neutron scattering for a few real binary compounds. Between the oldest known examples of this kind is the lattice dynamics of CuCl, semiconducting compound crystallizing in the zinc-blende structure. More recent data concern mercury chalcogenides: HgSe and β-HgS, also crystallizing in this structure. The lattice dynamics of all compounds above mentioned has been reproduced previously by the rigid-ion model calculations, in the case of CuCl and HgSe an excellent description of the phonon dispersion has also been achieved by the ab-initio calculations. However, a simple physical model pointing out the basic factors responsible for the “anomalous” optical phonon behavior has not been previously discussed in these papers.