Abstract
We have studied the ir reflectance spectra of epitaxially grown layers of Ga1-xAlxSb 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. © 1975 The American Physical Society.