Ferroelectric Relaxor Quantum Crystals

J Scott

doi:10.3390/cryst8040180

Abstract

A discussion is given of ferroelectrics (FEs) that have their Curie temperatures Tc very near absolute zero. These have differences in their dynamics in comparison with higher-temperature systems, since domain wall motion occurs via quantum mechanical tunneling and not by thermally activated diffusion. Emphasis in the present paper is on FEs that have relaxor characteristics. In such systems, the temperature at which the isothermal electric susceptibility ε(T,f) peaks is a strong function of frequency, and it decreases with decreasing frequency. This is due to glassy viscosity and is symbolic of non-equilibrium dynamics, usually described by a Vogel-Fulcher equation. It permits an extra dimension with which to examine the transitions. The second half of this paper reviews domain wall instabilities and asks about their presence in QCP ferroelectrics, which has not yet been reported and may be unobservable due to the absence of thermal diffusion of walls near T = 0; in this respect, we note that diffusion does exist in ferroelectric relaxors, even at T = 0, by virtue of their glassy, viscous dynamics.

Highlights

There are few simple stoichiometric ferroelectrics in which the Curie transition temperature from ferroelectric to paraelectric is very near absolute zero, but a few include potassium lithium tantalate (K3 Li2 Ta5 O15 ) at Tc = 7 K, lead pyrochlore at Tc = 15.4 K, and O-18 strontium titanate (Tc = ca. 35 K); in addition, sometimes substitutional methods can lower the transition by hundreds of degrees to zero
Measurements are made via dielectric constant and loss data, or by resonant ultrasonic attenuation (RUS), and in the former case, the frequency range of interest varies from a few Hz to a few MHz
Having presented the basic ideas about QCP ferroelectrics and, especially, QCP relaxors, let us turn to predictions of very specific differences in relaxor QCPs compared with other ferroelectric QCPs

Summary

Introduction

Examples include hexaferrites [1,2,3,4,5,6,7,8,9,10], tris-sarcosine calcium chloride [11], and a few others These systems are of general interest, but the interest rises with the degree of complexity, and one kind of additional complexity is provided by the glassy, viscous nature of some ferroelectrics. The amplitude of the dielectric constant increases; both this increase and the shift in response peak are typical of viscous materials, and a semi-empirical formula (Vogel-Fulcher Equation) is commonly used to fit the data This equation has an attempt frequency f0 for restricted ionic jumps, an activation energy Ea , a high-temperature onset

Results

Discussion

Conclusion