Experimental Studies On Quantum Ferroelectrics

The balance between dipolar forces and temporal fluctuations of the dipoles determines the temperature at which polar order sets in. If this temperature falls into the region of dominant zero-point vibration, the term “quantum ferroelectric” (QFE) is applied to denote the nature of the balance. To obtain this condition of quantum ferroelectricity, it is necessary to shift the Curie point into the desired temperature range. Experiments are reported for KDP-base QFE by application of hydrostatic pressure, and for KTaO3 by admixing isoelectronic impurities (Nb) during crystal growth (KTN = KTa1-xNbxO3). Dielectric, ultrasonic and pyroelectric measurements show the particularities of QFE’s: Their phase diagrams are given by Tc = (x-xc)1/2; x being pressure (KDP) or impurity concentration (KTN). The dielectric susceptibility varies as ε-1 - T2 for x = x, the polarization P - (x-x)1/2 at T = 0, and c c the elastic compliance is given by a step function. At temperatures outside the quantum range, T > 40 K, quantum ferroelectrics assume the properties of normal ferroelectrics. © 1981, Taylor & Francis Group, LLC. All rights reserved.