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.