DIELECTRIC STUDY OF THE PHASE-TRANSITION IN K1-XLIXTAO3

Comprehensive dielectric measurements have been performed on the mixed-crystal system K1-xLixTaO3 in order to check the hypotheses done about the ferroelectric and the dipole glass behaviours and identify their respective origins. These have been made for a series of concentrations (1.5% < x < 5%), spanning the critical concentration x(c) congruent-to 2%, and over the frequency range from 20 Hz to 2 MHz. The broad frequency and temperature ranges have allowed us to cover, in the same study, both Li-related dielectric relaxations as well as the transition for x > x(c) and to investigate their relationship. We have gathered new strong supports (quasi-discontinuity in the dielectric constant, thermal hysteresis) of first-order character of the transition for x = 3.5% and 5% and studied the kinetics within this hysteresis for x = 5%. We have also identified the high-temperature relaxation with an activation energy of about 2700 K, which we attribute to Li pairs undergoing pi flips at high temperatures. It is argued that, at low temperatures, these pairs are sources of static random fields responsible for the disorder of the low-temperature phase.