Relaxor ferroelectricity in the polar M2P-TCNQ charge-transfer crystal at the neutral-ionic interface

Abstract

We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (M 2 P-TCNQ), which is polar at room temperature and just at the neutral-to-ionic interface (ionicity ρ ≈ 0.5). We detect the typical dielectric signature of a relaxor ferroelectric and an asymmetric positive-up-negative-down behavior. While relaxor ferroelectricity is usually ascribed to disorder in the crystal, we find no evidence for structural disorder in the investigated crystals. To elucidate the origin of M 2 P-TCNQ's dielectric properties we perform parallel structural and spectroscopic measurements, associated with theoretical modeling and quantum-mechanical calculations. Our combined effort points to a highly polarizable electronic system that is strongly coupled to lattice vibrations. The found indications for polarization reversal imply flipping of the bent conformation of the M 2 P molecule with an associated energy barrier of a few tens of an eV, broadly consistent with an Arrhenius fit of the dielectric relaxation times. While the polarization is mostly of electronic origin, its possible reversal implies slow collective motions that are affected by solid-state intermolecular interactions.

TetramethylbenzidineTetrafluoroTCNQ (TMBTCNQF4): A Narrow-Gap Semiconducting Salt with Room-Temperature Relaxor Ferroelectric Behavior

“…Figure S1 shows that in the (||,||) polarization (red lines) the low-frequency modes are clearly enhanced with respect to the intra-molecular modes and lowfrequency modes in the (⊥,⊥) polarization (blue lines). However, this resonance effect is considerably lower than that observed in the previously studied dimerized stack CT crystal M 2 P-TCNQ [2].…”

Terahertz Raman scattering as a probe for electron–phonon coupling, disorder and correlation length in molecular materials

“…Although it may seem strange, a reliable comparison with calculations cannot be done, since for DA systems the calculation of the hopping (or CT) integrals strongly depends on the chosen basis set. 40 In our experience even a comparison between the ts of different DA pairs calculated with the same basis cannot be trusted. A recent study trying to circumvent the problem of direct calculation has been only partially successful, as it works well for quasi-neutral CT crystals 45 -not for M 2 P-TCNQ, for example.…”

“…Moreover, CT co-crystals can act as semiconductors, and although their room temperature mobility is two or three order of magnitudes lower than those of the best one-component systems, they can exhibit other, even more interesting, physical properties. [37][38][39] Indeed it was during a recent investigation of a polar CT co-crystal with relaxor ferroelectric properties 40 that pre-Resonance Raman spectroscopy put in evidence for the importance of Peierls coupling in that system. Here we shall extend the analysis, focusing on the theoretical basis of the observed effects, and showing the advantages and limitations of the approach.…”

“…with alternating long and short distances along the chain, justifying the treatment in terms of a collection of DA isolated pairs (dimer model). 40 Fig. 2 reports the T evolution of the THz Raman spectra with polarization (c,c), i.e., incident and scattered radiation polarized parallel to the stack axis.…”

“…2 shows that these bands also exhibit a considerable frequency softening (15-20 cm À1 ) by increasing T in the explored temperature range, a fact pointing to a strong degree of anharmonicity for the associated phonons. The bandwidth of the other low-frequency phonons is normal, 40 and so is their softening (top and bottom right panels of the The left panels of Fig. 3 show that by shifting the Raman exciting line towards longer wavelengths, namely, by going closer to the CT transition (located at about 5000 cm À1 ), the intensity of the 125 and 170 cm À1 group of bands in the (c,c) polarization is strongly enhanced with respect to that of the other ones, also in the different polarization.…”