Influence of orientational relaxation on the electrostrictive coupling in K [formula omitted]Li xTaO 3

Abstract

Polar regions are shown to mediate a strong coupling between polarization and strain in the paraelectric phase of the mixed ferroelectric K 1−x Li x TaO 3 (KLT) and KTa 1−x Nb x O 3 (KTN). This coupling is shown to result in a resonance in the dielectric spectrum of the crystals. In KLT, polar nanoregions can reorient via 180° (π-relaxation) or 90° (π/2-relaxation) rotations. While the π-relaxation is of no consequence, the π/2-relaxation has a strong influence on the overall character of the resonance. In addition to providing a mechanism for loss and degradation of the quality factor of the resonance, this relaxation alters its character from that of a resonance to that of a relaxation as the lithium reorientation frequency approaches and crosses the resonance. Experimental results from dielectric spectroscopy above and below this crossover are presented and discussed. Most significantly, the field strength needed to observe the polarization–strain coupling in KLT in the paraelectric phase is found to be 100 times smaller than it is needed for SrTiO 3 and BaTiO 3. This indicates that the electrostrictive coefficient is correspondingly 100 times larger in KLT and is due to the presence of polar nanoregions.