To elucidate a pressure-temperature phase diagram of the quasi-one-dimensional mixed-stack charge-transfer complex tetrathiafulvalene-$P$-chloranil (TTF-CA), we study the quasi-one-dimensional spin-1 Blume-Emery-Griffiths model. In addition to the local charge-transfer energy $\left(\Delta \right)$ and the inter-stack polar (dipole-dipole) interaction ${(J}_{\perp }),$ we take account of the interstack electrostriction (Coulomb-lattice coupling). Using the self-consistent chain-mean-field theory, where the intra-stack degrees of freedom are exactly treated by the transfer-matrix method, we reproduce the gas-liquid-solid like phase diagram corresponding to the neutral $\mathrm{}\left(N\right),$ paraelectric ionic ${(I}_{\mathrm{para}}),$ and ferroelectric ionic ${(I}_{\mathrm{ferro}})$ phases, respectively. Our classical model describes an essential point of the multicritical behavior of TTF-CA, i.e., the interchain electrostriction exclusively enhances the charge concentration (ionicity condensation), but does not affect the interchain ferroelectric coupling. This effect leads to appearance of the intermediate $I_{\mathrm{para}}$ phase in between the N and $I_{\mathrm{ferro}}$ phases on the $\Delta -T$ phase diagram.

• Received 12 July 2003

DOI:https://doi.org/10.1103/PhysRevB.69.075115