Tetramethylbenzidine–TetrafluoroTCNQ (TMB–TCNQF4): A Narrow-Gap Semiconducting Salt with Room-Temperature Relaxor Ferroelectric Behavior

We present an extension and revision of the spectroscopic and structural data of the mixed-stack charge-transfer (CT) crystal 3,3′,5,5′-tetramethylbenzidine–tetrafluorotetracyanoquinodimethane (TMB–TCNQF4), associated with new electric and dielectric measurements. Refinement of synchrotron structural data at low temperature has led to revise the previously reported C2/m structure. The revised structure is P21/m, with two dimerized stacks per unit cell, and is consistent with the low-temperature 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 P21/m─no phase transitions─but did not allow substantiating the presence of disorder. On the other hand, dielectric measurements have evidenced a typical relaxor ferroelectric behavior already at room temperature, with a peak in the real part of dielectric constant ϵ′(T,ν) 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 (Ea ∼ 0.3 eV) with a room-temperature conductivity of ∼10–4 Ω–1 cm–1.