Temperature ($T$) variations of the infrared molecular-vibration spectra of tetrathiafulvalene-chloranil (TTF-CA) crystalline powders have been measured at variable pressure ($P$). From the observed behavior of the spectra as a function of $T$ and $P$, it is concluded that both quasi-ionic and quasineutral molecules coexist in an intermediate range of $T$ and $P$, until the crystal undergoes a phase transition to the dimerized ionic phase at $T_c$ which depends on $P$. These results are discussed in terms of a $P-T$ phase diagram of TTF-CA crystal.

• Received 10 February 1987

DOI:https://doi.org/10.1103/PhysRevB.36.3884