Phonon and Exciton Amalgamation - - a Criterion F O R True Solid Solutions: Vibrations of Chemically and Isotopically Mixed Para-dihalobenzene Crystals*

The phonon and vtbratlonal exciton spectra are suggested as criteria for true solid solutions of molecular crystals. The short range character o f the interactions that determine both the p h o n o n and excl ton properties makes these bands ideal for distinguishing between truly random mLxed lattices and segremated microscopic domains. Raman studies o f the chemical13, mixed p-dichlorobenzene--p-d~bromobenzene crystal and the isotopically mixed p-dtchlorobertzened2a -p-d~chlorobenzene-d 4 crystals dlustrate the principles involved. All p h o n o n bands (external molecular vibrations) o f bo th the chemically and the isotopicaUy mixed crystals are in the amalgamation limit. VChde mass defects appear to determine the phonon frequency shifts in these mixed crystals, deviations f rom a virtual crystal model axe observed and discussed. The 1o~ energy internal modes (vibrational excitons) are examined and found to be in the separated hand limit. Treating the pure p-dichlorobenzene crystal as an isotopically mixed crystal due to the natural abundance o f the chlorine isotopes reveals tha t the chlorine s t retch at 310 cm r ts atso in the separated band limit. All these mixed crystal systems are concluded to be substitutionally random on the molecular scale.