Ferroelectric phase transition, ionicity condensation, and multicriticality in charge transfer organic complexes

TL;DR

This study models the pressure-temperature phase diagram of TTF-CA using a spin-1 BEG model, revealing complex phase transitions and multicritical behavior related to ferroelectricity and ionicity in charge-transfer complexes.

Contribution

It introduces a quasi-one-dimensional spin-1 BEG model including electrostriction to explain the phase diagram and multicriticality in TTF-CA, aligning theory with experiments.

Findings

Reproduces neutral, paraelectric ionic, and ferroelectric ionic phases.

Explains multicritical behavior and lattice contraction in TTF-CA.

Provides a theoretical framework for pressure-temperature phase transitions.

Abstract

To elucidate a novel pressure-temperature phase diagram of the quasi-one-dimensional mixed-stack charge-transfer (CT) complex TTF-CA, we study the quasi-one-dimensional spin-1 Blume-Emery-Griffith (BEG) model. In addition to the local charge transfer energy and the inter-stack polar (dipole-dipole) interaction, we take account of the inter-stack electrostriction effect. 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 (N), paraelectric ionic (I$_{\rm{para}}$), and ferroelectric ionic (I$_{\rm{ferro}}$) phases, respectively. We also give an explanation on the experimentally observed multicritical behavior and concomitant discontinuous inter-stack lattice contraction in TTF-CA.