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