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 xc ≅ 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 > xc 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 π 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.

In order to clarify the nature of the possible structural changes taking place in K1−xLixTaO3 (KLT), we have performed an ultrasonic study of three KLT crystals with approximate Li concentrations, 1.5%, 3.5% and 5%. A step increase in the velocity and a corresponding attenuation anomaly clearly indicate structural changes occurring at 30 K, 55 K and 75 K, respectively. In the two higher-concentration crystals, the large magnitude of this change and the observation of thermal hysteresis signal a macroscopic structural transition with a first-order character. In the lower-concentration crystal, a 3 to 5 times smaller effect and the absence of a hysteresis are attributed to local structural distortions without long-range order. This interpretation agrees with the conclusion of a recent Raman study of the same crystals. It provides a direct physical test of this conclusion which was derived from selection rules.

In order to describe the unusual dielectric properties observed in Ki1-χLiχTaO3. crystals a new approchh is proposed. The dynamical Glauber theory, previously applied to spinglasses, is modifiéd by the introduction of the spectral distribution of the random interactions between the dipoles associated with the Li+ ions. Moreover, the dipole corrélations are taken into account by the Onsager réaction field. As a result, the calculated dielectric constant reproduces well the unusual features of the Argand diagrams and, in particular, their finite slope at low frequencies and infinite slope at high frequencies (strophoidal shape). The temperature dépendance of some parameters shows, however, the limits of a spin-glass type model in describing the collective behaviour of randomly distributed dipoles in a highly polarizable medium

We have studied the relaxation of off-center Li + ions in KTaC>3 by dielectric measurements on 1% and 1.5% crystals, from 20 Hz to 2 MHz and from 4 to 200 K. The shape of the e" vs e Argand diagrams demonstrates the existence of a distribution of relaxation times. In analogy with spin glasses, a new expression is proposed for the analysis of these diagrams, a strophoidal function, which leads to a distribution function D(0) decreasing for large 6 as #~( 1+a) with 0 < a < 1. The most probable relaxation time 0 m p of the distribution follows an Arrhenius law with a barrier height close to 950 K.PACS numbers: 77.22.Gm, 64.60.-i When Li + ions are substituted for larger K + ions in KTaC>3 crystals they occupy off-center positions, therefore inducing an electric dipole moment as well as a nonspherical elastic quadrupole moment. Owing to the randomness of substitution, the interaction between dipoles is also random. This includes the usual dipolar interaction and the indirect coupling due to the soft ferroelectric mode in this highly polarizable crystal. The combination of these two types of interaction is expected to lead to unusual dipole dynamics and, upon cooling, to a phase transition, the nature of which is still not well understood [1,2]. In this Letter, we propose a new description of the dipole dynamics which is believed to play a crucial role in the transition.In order to investigate the relaxation of the Li dipoles, we have performed a systematic study of the complex dielectric constant s = e-ie" and the resulting Argand diagrams (AD), also called Cole-Cole diagrams, e" vs e. The shape of these diagrams clearly reveals the unusual character of the relaxation which we intend, in contrast to previous studies [3][4][5][6][7][8], to explain more precisely. To this end, we propose a new dielectric susceptibility function, the form of which was inspired by an analogy with the magnetic susceptibility of spin glasses. In both cases, the susceptibility is the response to an external field of an assembly of identically relaxing dipoles (magnetic or electric) when the interactions between dipoles are random. The eigenmodes of the system may be determined by diagonalization of the interaction matrix. The corresponding transformation leads to a distribution of relaxation times for the eigenmodes which, in turn, determines the form of the dynamic susceptibility [9,10].The dielectric measurements have been carried out on two crystals of Ki-x Li x Ta03 (KLT) with concentrations JC = 1% and 1.5% as a function of the temperature (4 K < T < 200 K) and over a broad range of frequency (20 Hz