Acoustic and dielectric studies of lithium-potassium tantalate crystals

In the mixed crystal K1 - xLixTaO3 the off-center Li+ ions carry an electric dipolar moment, coupled dipolar moment, coupled to electric fields, and an elastic quadrupolar moment, coupled to elastic strains. We have performed acoustic and dielectric measurements, in crystals with x = 1.7% (Tc = 35 K) and x = 5% (Tc = 75 K). In particular, we have studied the relaxation of the Li+ ions, which manifests itself on both the dielectric constant and the elastic constant by a peak in the imaginary part and a step in the real part. In the less concentrated crystal, the dielectric results are analyzed in the form of Argand (Cole-Cole) diagrams exhibiting a very particular (strophoidal) shape, from which an unusual and temperaturedependent distribution of the relaxation times is deduced. In the second crystal we have studied the time-dependent effects, the effect of the cooling-rate and the thermal hysteresis under Tc. All features indicate a first-order-like transition, which is clearly observed by both techniques.