A new type of neutral-ionic interface in mixed-stack organic charge-tranfer crystals: Temperature induced ionicity change in ClMePD-DMeDCNQI

摘要

Raman and polarized infrared spectra of the mixed stack charge transfercrystal 2-chloro-5methyl-p-phenylendiamine- -2,5-dimethyl-dicyanoquinonediimine(ClMePD-DMeDCNQI) are reported as a function of temperature. A detailedspectral interpretation allows us to gain new insight into the temperatureinduced neutral-ionic transition in this compound. In particular, the crossingof the neutral-ionic borderline appears to be quite different from that of thefew known temperature induced neutral-ionic phase transitions. First of all,the ionicity change is continuous. Furthermore, the onset of stack dimerizationprecedes, rather than accompanies, the neutral-ionic crossing. The (secondorder) phase transition is then driven by the dimerization, but the extent ofdimerization is in turn affected by the ionicity change.