Dielectric constant, loss factor and ac conductivity of Ni2+-doped K2ZnCl4 crystals in the ferroelectric-commensurate, incommensurate and normal phases

Abstract

The dielectric constant (ϵ), dielectric loss (tanδ) and ac conductivity (σac) of virgin and thermally cycled undoped and Ni2+-doped K2ZnCl4 (KZC) crystals in the ferroelectric-commensurate (FC), incommensurate (IC) and normal phases (N) have been studied. Anomalous behaviour at the two-phase transition points was observed while measuring ϵ along the polar a-axis for the undoped sample. With increasing Ni2+concentration a systematic shift of the phase transition temperature towards lower values and a continuous inhibition of the peak height were detected. ϵ changed linearly with lnf up to f =105 Hz. tanδ along the a-axis declared the phase transitions by peak changes. After Ni2+-doping this behaviour was preserved at the FC-IC phase transition point while the IC-N phase transition was manifested by a change in the slope of the straight line representing the tanδ-T dependence. The ac conductivity changed lineally with frequency according to a relation of the form σac = σof β where 0>β>1.9. σac increased monotonically with increasing temperature and doping concentration in the low-temperature phases tending to merge in one straight-line with high activation energy that might be due to superionic dc conduction in the high temperature N-phase. Doping with Ni2+ pinned stripples, decreased the soliton-soliton interaction, weakened discommensuration effects, shortened the IC-phase and strongly affected the ‘chaotic' behaviour at the TC-IC. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)