Effects of molecular substitution on the neutral-ionic (NI) phase transition have been investigated for tetrathiafulvalene (TTF)-p-chloranil $\left({\mathrm{QCl}}_4\right)$ complex doped with tetraselenafulvalene (TSF) or trichloro-p-benzoquinone $\left({\mathrm{QCl}}_3\right)$ molecules. Except for the fully substituted compound ${\mathrm{T}\mathrm{T}\mathrm{F}-\mathrm{Q}\mathrm{C}\mathrm{l}}_3$ 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 ${\mathrm{QCl}}_3$ 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 ${\mathrm{QCl}}_3$ which yields binary-phase separation into ferroelectric (ionic) and paraelectric (neutral) regions.

• Received 16 October 1998

DOI:https://doi.org/10.1103/PhysRevB.59.11267