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

We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (M2P-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 M2P-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 M2P 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.