Effect of nonrandomness on long-wavelength optical phonons inZnS1−xSex

A concentration-dependent model using the effect of nonrandomness on long-wavelength optical phonons in a mixed crystal $A{B}_{1-x}{C}_x$ has been investigated. The main features of the model are (1) a mixing parameter $\lambda$ to describe the interaction between atoms $B$ and $C$, (2) two-mode behavior in general, reducing to one-mode behavior at $\lambda =0\mathrm{}$, and (3) only two optical frequencies for exact $x=0\mathrm{}$ and 1 in the case of diatomic crystals. The model has been applied to $\mathrm{Zn}{\mathrm{S}}_{1-x}{\mathrm{Se}}_x$ to investigate the phonon dispersion curves in the symmetry directions. The results for $x=0, 0.2, \mathrm{and} 1.0$ are found to be in agreement with available experimental values.