THE FERROELECTRIC AND PROTON GLASS COEXISTENCE REGION IN K-0.77(NH4)(0.23)H2ASO4 DETECTED BY COMPLEX PERMITTIVITY MEASUREMENTS

This paper reports results of the studies on complex permittivity epsi lon = epsilon' - i epsilon'' in a mixed K-0.77(NH4)(0.23)H2AsO4 cryst al. Measurements along the crystallographic a axis have clearly indica ted the coexistence of the ferroelectric state and proton glass state in this crystal. The temperature dependence of epsilon'(a) shows a max imum at T-c = 61 K, indicating a ferroelectric transition, while the d ispersion of epsilon'(a) and E epsilon''(a) below T = 40 K satisfies t he Vogel-Fulcher equation with the parameters T-0 = 8.3 K, E(c) = 183. 5 K and v(c) = 1.65 x 10(11) Hz, which proves the presence of the prot on glass state. Analysis of the shape of the temperature dependence of dielectric losses epsilon''(c)(v, T) observed in the measurements alo ng the ferroelectric c axis revealed the presence of another process o f relaxation apart from that related to the freezing of ferroelectric domain wall motion known for KDP type crystals. The two relaxation pro cesses are also described by the Vogel-Fulcher equation with the param eters T-01 = 34 K, E(c1) = 7.7 K and v(c1) = 6.8 x 10(5) Hz for one of them and for the other T-02 = 20 5 K, E(c2) = 13.4 K and v(c2) = 5.6 x 10(6) Hz. Our results prove the coexistence of the ferroelectric and proton glass states in the K-0.77 (NH4)(0.23)H2AsO4 crystal, and the occurrence of different kinds of cluster of local ordering (proton gla ss and relaxer clusters) and ferroelectric domains.