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Temperature-Induced Neutral-Ionic Phase Transition in the (EDT-TTF-I2)2TCNQF Mixed-Stack Charge-Transfer Salt

Abstract : We report the infrared (IR) and Raman study of the 2:1 mixed-stack charge transfer salt (EDT-TTF-I2)2TCNQF, where EDT-TTF = ethylenedithiotetrathiafulvalene and TCNQ = tetracyanoquinodimethane, which undergoes a temperature-induced neutral-ionic phase transition. Polarized infrared (IR) and Raman spectra of single crystals were measured in the 8-293 K temperature range. Temperature variations of vibrational modes that are assigned to both donors and acceptors show that the average degree of charge-transfer is growing continuously from about zero at room-temperature to about 1e at T = 8 K; nevertheless, at about 100 K a regime change is observed. The coexistence of molecular species of different ionicity is detected in the whole temperature range. The IR vibrational features due to electron-molecular vibrational coupling increase their intensities considerably upon cooling giving an evidence of relatively small gradual distortions of crystal structure. Existence of ferroelectric domains is suggested. A scheme of the neutral-ionic phase transition in a 2:1 complex is proposed and discussed. © 2016 American Chemical Society.
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A. Frackowiak, I. Olejniczak, R. Świetlik, O. Jeannin, M. Fourmigué. Temperature-Induced Neutral-Ionic Phase Transition in the (EDT-TTF-I2)2TCNQF Mixed-Stack Charge-Transfer Salt. Journal of Physical Chemistry C, American Chemical Society, 2016, 120 (41), pp.23740--23747. ⟨10.1021/acs.jpcc.6b07642⟩. ⟨hal-01398047⟩

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