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arXiv · 2104.04454

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

Stefano Canossa·Elena Ferrari·Pit Sippel·Jonas K. H. Fischer·Raphael Pfattner·Ruggero Frison·Matteo Masino·Marta Mas-Torrent·Peter Lunkenheimer·Conceptió Rovira·Alberto Girlando
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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 $P2_1/m$, 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 $P2_1/m$ -- 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 $\epsilon'(T,\nu)$ 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 $\sim 10^{-4}~ \Omega^{-1}$ cm$^{-1}$.

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Stefano Canossa, Elena Ferrari, Pit Sippel, Jonas K. H. Fischer, Raphael Pfattner, Ruggero Frison, Matteo Masino, Marta Mas-Torrent, Peter Lunkenheimer, Conceptió Rovira, Alberto Girlando. 2021-04-09. Tetramethylbenzidine-TetrafluoroTCNQ: A narrow-gap semiconducting salt with room temperature relaxor ferroelectric behavior. https://doi.org/10.1021/acs.jpcc.1c07131

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