Long‐Wavelength Optical Phonons in Mixed Crystals. II. The persistence of common gaps

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Abstract
The formalism developed in Part I in order to describe the long‐wavelength optical phonons in mixed crystals as a system of two coupled modes is now applied to the two mixed systems In1–xGaxP and GaAs1–xPx as representative ones for one‐ and two‐mode behaviour, respectively. Calculated mode frequencies, eigenvectors, and infrared oscillator strengths are presented and their behaviour is discussed in dependence on composition in both mixed systems. Contrary to earlier suggestions we state that in a traditionally named one‐mode system apart from the lattice‐like mode in addition an impurity‐like mode occurs which is always an in‐band mode. A two‐mode system is characterized by a gap between the two sets of optical phonons arising from a gap between the optical phonon bands of the end‐member crystals. A new criterion is given for one‐ or two‐mode behaviour in terms of an overlapping or the existence of a gap between the optical phonon densities of states of the end‐member crystals. It is suggested from several items that the persistence of common gaps of the end‐member crystals is a determining factor in the lattice dynamics of mixed systems.