Long wavelength optical phonons in mixed crystals

Experimental data on optical phonons in mixed crystal systems obtained by infra-red, Raman, and other optical measurements are reviewed. In terms of their spectral behaviour, mixed crystals may be classified into different types, using as criteria the mass ratios of the constituent atoms and the inter-atomic forces. Several phenomenological models have so far been proposed for the explanation of the mixed crystal behaviour; these are reviewed. The difficulties facing a realistic and thorough lattice dynamical calculation of a disordered system are pointed out. Further experimental work needed for confirming and gaining understanding of the true behaviour types of some mixed crystal systems is suggested. © Taylor & Francis Group, LLC.