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We investigated the mixed-stack charge-transfer crystal,N,N'-dimethylphenazine-TCNQ (M$_2$P-TCNQ), which is polar at room temperatureand just at the neutral-to-ionic interface (ionicity $\rho \approx 0.5$). Wedetect the typical dielectric signature of a relaxor ferroelectric and anasymmetric positive-up-negative-down behavior. While relaxor ferroelectricityis usually ascribed to disorder in the crystal, we find no evidence forstructural disorder in the investigated crystals. To elucidate the origin ofM$_2$P-TCNQ's dielectric properties we perform parallel structural andspectroscopic measurements, associated with theoretical modeling andquantum-mechanical calculations. Our combined effort points to a highlypolarizable electronic system that is strongly coupled to lattice vibrations.The found indications for polarization reversal imply flipping of the bentconformation of the M$_2$P molecule with an associated energy barrier of a fewtens of an eV, broadly consistent with an Arrhenius fit of the dielectricrelaxation times. While the polarization is mostly of electronic origin, itspossible reversal implies slow collective motions that are affected bysolid-state intermolecular interactions.

Intriguing relaxor ferroelectricity in a polar CT crystal at the neutral-ionic interface