Calculation of excess free energy of mixing and analysis of phonon spectra with clustering parameter in III-V pseudobinary alloys

The excess free energy of mixing, ΔG/sup ex/, is calculated and the phonon spectra for III-V pseudobinary alloys A/sub 1-x/B/sub x/C are analyzed on the basis of a cluster model. ΔG/sup ex/ is a function of the clustering parameter β which represents the deviation from the random atomic distribution The value of β giving the minimum excess free energy, β/sub s/, takes the maximum value at x=0.5 and it increases with increase in the relative difference of lattice constants, Δa/a-bar.The value of β giving the best agreement between the measured phonon frequencies and the calculated ones, β/sub ph/, takes the maximum value also at about x=0.5 in systems exhibiting the phonon spectra of partly-two mode type. The value of β/sub s/ at x=0.5 and β/sub ph/ over the entire composition range is zero for two-mode systems, such as (AlGa)-V. (author)