Influence of orientational relaxation on the electrostrictive coupling in K1-xLixTaO3

Polar regions are shown to mediate a strong coupling between polarization a nd strain in the paraelectric phase of the mixed ferroelectric K1-xLixTaO3 (KLT) and KTa1-xNbxO3 (KTN). This coupling is shown to result in a resonanc e in the dielectric spectrum of the crystals. In KLT, polar nanoregions can reorient via 180 degrees (pi-relaxation) or 90 degrees (1 pi/2-relaxation) rotations. While the rr-relaxation is of no consequence, the pi/2-relaxati on has a strong influence on the overall character of the resonance. In add ition to providing a mechanism for loss and degradation of the quality fact or of the resonance, this relaxation alters its character from that of a re sonance to that of a relaxation as the lithium reorientation frequency appr oaches and crosses the resonance. Experimental results from dielectric spec troscopy above and below this crossover are presented and discussed. Most s ignificantly, 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 SrTiO3 and BaTiO3. This indicates that the electrostr ictive coefficient is correspondingly 100 times larger in KLT and is due to the presence of polar nanoregions. (C) 1999 Elsevier Science Ltd. All righ ts reserved.