POLARIZATION-STRAIN COUPLING IN THE MIXED FERROELECTRIC KTA1-XNBXO3

The peculiar polar properties of KTa1-xNbxO3 (KTN) are determined by the presence of electric dipoles associated with randomly distributed Nh off-center ions. Because the latter also possess an elastic quadrupole, they should permit a coupling between electric polarization and strain. In order to investigate this coupling, we have carried out an ultrasonic study of KTN for different Nh concentrations. The elastic constants C-11 and C-44 and the corresponding attenuations have been measured in the presence of de bias electric fields of varying strengths. These fields were also applied in different directions relative to the acoustic polarization of the ultrasonic waves. The present ultrasonic results confirm the formation of polar nanoregions at T*congruent to T-c+20 K, marking the transition to a strong-coupling regime. They can be described by a microscopic model in which the primary polarization fluctuations are associated with the collective reorientations of the Nb off-center ions within a polar nanoregion. These fluctuations and their coupling to the sound waves can be enhanced or depressed by a de bias electric field. An E(2) dependence of the elastic constant indicates the electrostrictive nature of the coupling.