Long-wavelength optic phonons inGa1−xAlxP

We have measured the ir reflectance spectra of the alloy system ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{P}$ and found two-mode behavior for the long-wavelength optic phonons. We have analyzed the compositional dispersion of the TO phonons using the random-element-isodisplacement (REI) model. Comparisons of the REI force constants for ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{P}$ with those obtained for ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{Sb}$ and ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{As}$ indicate that the model second-neighbor force is not a measure of the real second-neighbor interactions.