Ultrasonic study of mixed ferroelectrics in a DC bias field

Polarization and Raman measurements, in KTa_1-XNb_X O₃ (KTN), have shown the existence of precursor effects many degrees above the transition. These effects are due to the formation of polar nanoregions, or clusters, which also explain the departure of the susceptibility from its Curie-Weiss dependence at high temperature. It was also shown through ultrasonic measurements at high temperatures and concentrations that the clusters behave as dynamical polar fluctuations resulting in a softening of C₁₁ with a (T-T_c)^-0.5 dependence. In order to investigate the coupling between these polar fluctuations and the elastic strains of the solid, measurements of the elastic constants have been made for applied DC fields in the direction of propagation. Results are presented for C ₁₁ and C₄₄ in concentrations up to 16% Nb. These measurements have been extended through the transition where evidence has been found to support the existence of three transitions even at the lowest concentration