Optical phonons with a negative oscillator strength

We discuss the nature of additional (redundant from the point of view of selection rules) optical phonons observed in most crystals with ion–covalent bonds between atoms, including in their solid solutions. These ‘redundant’ phonons are located in the frequency range of the longitudinal–transverse splitting of fundamental phonons, where the real part of the crystal permittivity is negative. They are also split by the crystal field into transverse and longitudinal phonons such that the frequencies of redundant longitudinal phonons are lower than those of redundant transverse phonons and the oscillator strength of these phonons is negative.