The ferroelectric and proton glass coexistence region in K0.77(NH4)0.23H2AsO4 detected by complex permittivity measurements

Abstract

This paper reports results of the studies on complex permittivity epsilon *= epsilon '-i epsilon " in a mixed K0.77(NH4)0.23H2ASO4 crystal. Measurements along the crystallographic a axis have clearly indicated the coexistence of the ferroelectric state and proton glass state in this crystal. The temperature dependence of epsilon 'a shows a maximum at Tc=61 K, indicating a ferroelectric transition, while the dispersion of epsilon 'a and epsilon a" below T=40 K satisfies the Vogel-Fulcher equation with the parameters T0=8.3 K, Ec=183.5 K and nu c=1.65*1011 Hz, which proves the presence of the proton glass state. Analysis of the shape of the temperature dependence of dielectric losses epsilon c"( nu ,T) observed in the measurements along the ferroelectric c axis revealed the presence of Another process of relaxation apart from that related to the freezing of ferroelectric domain wall motion known for KDP type crystals. The two relaxation processes are also described by the Vogel-Fulcher equation with the parameters T01=34 K, Ecl=7.7 K and nu cl=6.8*105 Hz for one of them and for the other T02=20.5 K, Ec2=13.4 K and nu c2=5.6*106 Hz. Our results prove the coexistence of the ferroelectric and proton glass states in the K0.77(NH4)0.23H2AsO4 crystal, and the occurrence of different kinds of cluster of local ordering (proton glass and relaxer clusters) and ferroelectric domains.