Phonon and exciton amalgamation - A criterion for true solid solutions: Vibrations of chemically and isotopically mixed para-dihalobenzene crystals

Abstract

The phonon and vibrational exciton spectra are suggested as criteria for true solid solutions of molecular crystals. The short range character of the interactions that determine both the phonon and exciton properties makes these bands ideal for distinguishing between truly random mixed lattices and segregated microscopic domains. Raman studies of the chemically mixed p-dichlorobenzen-p-dibromobenzene crystal and the isotopically mixed p-dichlorobenzene-h4-p-dichlorobenzene-d4 crystals illustrate the principles involved. All phonon bands (external molecular vibrations) of both the chemically and the isotopically mixed crystals are in the amalgamation limit. While mass defects appear to determine the phonon frequency shifts in these mixed crystals, deviations from a virtual crystal model are observed and discussed. The low energy internal modes (vibrational excitons) are examined and found to be in the separated band limit. Treating the pure p-dichlorobenzene crystal as an isotopically mixed crystal due to the natural abundance of the chlorine isotopes reveals that the chlorine stretch at 310 cm−1 is also in the separated band limit. All these mixed crystal systems are concluded to be substitutionally random on the molecular scale.