We have studied the ir reflectance spectra of epitaxially grown layers of ${\mathrm{Ga}}_{1-x}{\mathrm{Al}}_x\mathrm{Sb}$ and have found two-mode behavior for the long-wavelength optic phonons. We have analyzed the compositional variation in the TO-phonon frequencies using force constants derived from the random-element-isodisplacement model. We find that the data are well described by a force-constant model that includes six parameters, three force constants each with a different compositional dependence. For comparison, we have also analyzed the data using other force-constant representations including four-parameter models in which the same compositional dependence is assumed for each of the forces.

• Received 31 March 1975

DOI:https://doi.org/10.1103/PhysRevB.12.4135