VIBRATIONAL PROPERTIES OF A 2D ISING ALLOY

The effects of short-range order on phonons in pseudobinary solid solu tions A(x)B(1-x)C were studied for the first time by means of the dire ct diagonalization of a dynamical matrix corresponding to the equilibr ial distribution of impurity atoms. For two-mode alloys it is shown th at ordering in the sublattice with impurity produces extra modes of th e spectral density of phonon states while clustering mainly shifts the fundamental modes upwards in frequency. We also show that in some cas es clustering may affect the (one/two-mode) type of optical spectra. W e compare qualitatively our results to available experimental data for semiconducting mixed crystals.