Dielectric properties ofRb2ZnCl4crystals in the commensurate phase

Dielectric spectroscopy data in the frequency range 100 Hz–10 MHz are presented for two kinds of ${\mathrm{Rb}}_2$ ${\mathrm{ZnCl}}_4$ crystals, purified and standard quality, with special attention to the lock-in ferroelectric phase (C phase). It is shown that unlike standard-quality samples, the permittivity ɛ tail extends deep into the C phase of purified crystals and is an almost unambiguous function of temperature. The ɛ(T) dependence and its dispersion properties show that the temperature region below the incommensurate (IC) phase can be divided into four sections. On cooling, the first-order transformation takes place within the interval Δ${\mathit{T}}_{\mathit{c}}$ in which dɛ/dT0. In Δ${\mathit{T}}_2$ a Debye-like relaxation dominates. The mean relaxation time fulfills the Arrhenius law and the wall-motion mechanism is believed to be thermally activated. Below, in the interval Δ${\mathit{T}}_3$, it is replaced by another mechanism characterized by low-frequency relaxation tails extending below the measuring frequency range.