We investigated the mixed-stack charge-transfer crystal, N,N'-dimethylphenazine-TCNQ (${\mathrm{M}}_2\mathrm{P}$-TCNQ), which is polar at room temperature and just at the neutral-to-ionic interface (ionicity $\rho \approx 0.5$). 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 ${\mathrm{M}}_2\mathrm{P}$-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 ${\mathrm{M}}_2\mathrm{P}$ 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.

中文翻译：

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中性离子界面处的PolarM2P-TCNQ电荷转移晶体中的弛豫铁电

我们研究了混合堆叠电荷转移晶体N，N'-二甲基吩嗪-TCNQ（${\mathrm{中号}}_{\mathrm{2个}}\mathrm{P}$-TCNQ），它在室温下是极性的，并且在中性与离子间的界面（离子性 $\rho \approx 0.5$）。我们检测到弛豫铁电的典型介电特征和不对称的正-负-负行为。尽管弛豫铁电通常归因于晶体中的无序，但我们没有发现所研究晶体中结构无序的证据。阐明${\mathrm{中号}}_{\mathrm{2个}}\mathrm{P}$-TCNQ的介电性能，我们执行了平行的结构和光谱测量，并与理论建模和量子力学计算相关联。我们的共同努力指向了高度极化的电子系统，该系统与晶格振动紧密耦合。极化反转的发现迹象表明，弯曲的构象发生了翻转。${\mathrm{中号}}_{\mathrm{2个}}\mathrm{P}$具有数十eV的相关能垒的分子，与介电弛豫时间的Arrhenius拟合大致一致。尽管极化主要是电子起源的，但其可能的逆转意味着受固态分子间相互作用影响的缓慢的集体运动。