Thermal hysteresis phenomena and mesoscopic phase coexistence around the neutral-ionic phase transition in TTF-CA and TMB-TCNQ

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Abstract
Hysteresis phenomena are investigated around the temperature-induced neutral-ionic (N-I) phase transition in the prototype compound tetrathiafulvalene-p chloranil (TFF-CA). A multistep transition is evidenced by neutron scattering, with plateau evolution of Bragg peaks characteristic of symmetry breaking, as well as by specific heat measurements revealing a multipeak structure, both in perfect agreement. A structural study reveals the coexistence of N and I phases at a mesoscopic scale during the thermal hysteresis. No gradual process with intermediate long periodicity structure based on alternating N and I planes, as theoretically suggested, takes place. These results, supported by optical microscopy studies performed on both TTF-CA and tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ) crystals, underline the importance of interstack elastic coupling at the temperature-induced neutral-ionic transition.