Effect of chemical doping on the ferroelectric neutral-ionic phase transition in tetrathiafulvalene-p-chloranil (TTF-QCl4)

Effects of molecular substitution on the neutral-ionic (NI) phase transition have been investigated for tetrathiafulvalene (TTF)-p-chloranil (QCl(4)) complex doped with tetraselenafulvalene (TSF) or trichloro-p-benzoquinone (QCl(3)) molecules. Except for the fully substituted compound TTF-QCl(3) of minor structural modifications, x-ray-diffraction measurements confirm the isostructurality over a wide compositional range as well as smooth change in the lattice constants by molecular replacement. With increasing TSF concentration, the peak in the dielectric constant corresponding to the ferroelectric NI transition smoothly shifts toward zero temperature due to the lattice expansion. Around the critical concentration are found some characteristics of quantum ferro(para)electricity. Although the increment of QCl(3) concentration also lowers the peak temperature of the dielectric constant, the dielectric anomalies reveal glasslike frequency dependence. We attributed the glasslike behavior to the strong "pinning-impurity'' effect of QCl(3) which yields binary-phase separation into ferroelectric (ionic) and paraelectric (neutral) regions. [S0163-1829(99)03617-6].