Tetramethylbenzidine-TetrafluoroTCNQ: A narrow-gap semiconducting salt with room temperature relaxor ferroelectric behavior

Abstract

We present an extension and revision of the spectroscopic and structural data of the mixed stack charge transfer (CT) crystal 3,3${}^{\prime }$,5,5${}^{\prime }$-tetramethylbenzidine--tetrafluoro-tetracyanoquinodimethane (TMB-TCNQF4), associated with new electric and dielectric measurements. Refinement of syncrotron structural data at low temperature has led to revise the previously reported [Phys. Rev. Mat. 2, 024602 (2018)] $C2/m$ structure. The revised structure is $P{2}_1/m$, with two dimerized stacks per unit cell, and is consistent with the vibrational data. However, polarized Raman data in the low-frequency region also indicate that by increasing temperature above 200 K the structure presents an increasing degree of disorder mainly along the stack axis. X-ray diffraction data at room temperature have confirmed that the correct structure is $P{2}_1/m$ -- no phase transitions -- but did not allow to definitely substantiate the presence of disorder. On the other hand, dielectric measurement have evidenced a typical relaxor ferroelectric behavior already at room temperature, with a peak in real part of dielectric constant $\hat{I}{\mu }^{\prime }(T,\hat{I}½)$ around 200 K and 0.1 Hz. The relaxor behavior is explained in terms of the presence of spin solitons separating domains of opposite polarity that yield to ferroelectric nanodomains. TMB-TCNQF4 is confirmed to be a narrow gap band semiconductor ($E_a\sim 0.3$ eV) with room temperature conductivity of $\sim 10^{-4}\hat{I}{\text{c◯}}^{-1}$ cm${}^{-1}$.

21 pages, including the Supporting Information in the same file. Version 3 updates the x-ray structural data at room temperature